RIA's have been made for progesterone and estrone with the antisera S74B7 and 7604-7 40, respectively, which will be described. The characteristics of the RIA for progesterone resemble those of other reported RIA's. The antiserum for the RIA of estrone is highly specific, with main cross-reactions of 0.32%. 0.27% and 0.13% for estradiol-17P, estradioLl7a and oestriol, respectively. The RIA for estrone is sensitive (affinity constant: 28.2 x 10" l/mol), with a standard curve in the range from 1 to 10 pg. Accuracy and precision were reliable, as proved by the significant correlation between steroid added and steroid estimated.With the use of these RIA's. progesterone and estrone were demonstrated in the ovaries and pyloric ceca of single specimens of Asterias rubens. Progesterone was also present in the perivisceral fluid, but estrone appeared to be absent. The progesterone level was lower and the estrone level higher in the ovaries than in the pyloric ceca.The significance of the presence of these steroids in the ovaries and pyloric ceca of Asrerias rubens,

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Oocyte diameters and their frequency distribution, and various other data determined for the oestradiol-l7P treated female specimens of Asterias rubens proved to be significantly different from those for the control animals. The maturation index of the treated animals is 2.4, that of the control animals 1.4. Since the treated animals show a greater heterogeneity in development than the control animals, and because the diameter of the smallest oocytes is the same for both treated and control animals, a threshold size of the oocytes may be required before oocyte growth can be stimulated by oestradiol-l7P, and before substances originating in the pyloric caeca are incorporated into the oocytes. Oestradiol-17P treatment caused a tenfold increase of the oestrone level in the ovaries, whereas a non-significant increase was observed in the pyloric caeca. This may indicate that in vivo oestradiol-17,B is converted into oestrone in the ovaries but not in the pyloric caeca.Recently, biosynthesis of steroids has been proved to take place in the ovaries and pyloric caeca of the starfish Asterias rubens. The results of in vitro incubation experiments provided evidence of the production of C,,-steroids from cholesterol') and of C,,-steroids from progesterone (9, 10, 14). Although a high metabolic activity of C,,-steroids has been noticed (1 l), evidence for the formation of oestrogens from androstenedione had not yet been obtained.Coinciding with the time of increased activity of 3P-hydroxysteroid dehydrogenase (3P-HSD) (12, 17) -a parameter for the steroid synthesizing capacity -cells with ultrastructural features characteristic of steroid synthesizing cells, have been observed in the ovaries near the haemal system (1 5, 16). Radioimmuno assays (RIA'S) indicated progesterone, oestradiol-17P and oestrone to be present in the ovaries and pyloric caeca of female specimens of Asterius rubens (3, 13, 17). Variations in the progesterone level in the ovaries, related to morphological and biochemical data, suggested a function of progesterone during the growth of oocytes. The peak level of oestrone in the ovaries at the end of the resting stage (stage 1) of the annual reproductive cycle coincides with a maximal protein content in the pyloric caeca and is followed by a peak level of protein in the ovaries (13, 17). These findings lead to the hypothesis that i , Steroid nomenclature: cholesterol =3P-hydroxycholest-5-ene progesterone = pregn-4-ene-3, 20-dione oestrone = 3-hydroxyestra-l , 3, 5( lO)-triene-l7-0ne oestradiol-17n = 3, 17a-dihydroxyestra-1, 3, 5(10)-triene oestradioLl7P = 3, 17,5'-dihydroxyestra-l, 3, 5(10)-triene oes t riol =3, 16a, 178-trihydroxyestra-1, 3, 5(10)-triene

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