NOTE 325and biological characters (9). They do have common antigens, as shown by double immunodiffusion (7), but cross-reactions have never been demonstrated by growth inhibition and immunofluorescence, and only weak or oneway cross-reactions have ever been found by metabolism inhibition. Considering the species concept of mycoplasmas in general and the homology levels cited above, it would not seem justifiable to classify the Donetta-like strains as a subspecies of M . agalactiae but rather as a separate species, with the type strain Donetta

The base compositions of the deoxyribonucleic acid and the genome sizes of eight serotypes of human T-mycoplasmas are reported. The guanine plus cytosine contents, as measured by thermal denaturation and CsCl gradient centrifugation, are 27 t o 28%, the latter method iving slightly lower values. The

Within the order Mycoplasmatales guanine plus cytosine contents and genome sizes have been determined for 17 strains representing 15 bovine and two ovine species, subspecies or serogroups. The guanine plus cytosine contents were found to vary between 24-4 and 32.9 %. The genome sizes were 4.0 to 5-6 x 1oS daltons for 14 sterol-requiring strains and 0.99 to 1.1 x 109 daltons for three non-sterol-requiring strains. These results support earlier findings that members of the genus Acholeplasma (family Acholeplasmataceae) have genome sizes about twice those of the genus Mycoplasma (family Mycoplasmataceae). The purpose of this work was to determine the GC contents and the genome sizes of type or reference strains of all known mycoplasma species, subspecies and serogroups of bovine source. The type strains (PG2 and PG3) of two ovine organisms (Mycoplasma agalactiae subsp. agalactiae and M. mycoides subsp. Capri) were included in the study because of serological relatedness to the bovine strains ( M . agalactiae subsp. bovis (Donetta) and M . mycoides subsp. mycoides (PGI), respectively). METHODSThe mycoplasmas investigated (Table I) The substrates used for cultivation were in most cases Bacto Heart Infusion Broth (Difco), supplemented with 15 % horse serum or I % PPLO Serum Fraction, 8 % fresh yeast extract, 0.003 % phenol red, 0.08 % thallium acetate and penicillin (400 i.u./ml). Some strains were also supplied with 0.8 % glucose (PG49, PG50, B107PA7 PGI, PG3) or I % arginine (B139P7 B I~~P , ~230). Mycoplasma dispar was cultivated in FF I1 medium (Friis, 1972), with minor modifications; it comprised Bacto Brain Heart Infusion Broth (Difco, Detroit, Michigan, U.S.A.),

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