A highly sensitive antiglobulin test based on rosette formation due to the interaction between IgG bearing red blood cells (RBC) and Fc receptors on K562 cells, was used to study the immunoglobulin molecules present on human senescent RBC. Normal human RBC were separated into young and senescent subpopulations on the basis of age-dependent differences in density by centrifugation on a discontinuous density Percoll gradient, and by flotation on phthalate ester mixtures. The senescent but not the young RBC were found to bear membrane bound IgG. Most of the bound IgG molecules could be specifically eluted by galactose in its alpha-anomeric form. Antigalactosyl (anti-Gal) IgG antibodies with similar reactivity were found to be present in high titres in every one of the 400 normal human sera tested. The natural anti-Gal antibodies isolated from normal sera by affinity chromatography could bind to IgG depleted senescent RBC but not to young RBC. Erythrophagocytosis experiments indicated that the anti-Gal bound to the senescent RBC induced their destruction by macrophages. It is suggested that the natural anti-Gal antibodies interact with cryptic alpha-galactosyl residues which are exposed in the course of the RBC senescence and mediate the removal of these RBC from circulation by cells of the reticuloendothelial system.

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SUMMARYNuclease activity was observed in several saprophytic and parasitic Mycoplasma organisms ; the nucleases of Mycoplasma laidlawii were studied in detail. Nuclease activity of this organism was highest a t the early logarithmic phase of growth, and was found mainly in the soluble fraction of the organisms. Anion-exchange chromatography of the proteins of the soluble fraction separated ribonuclease from deoxyribonuclease activity. Each enzyme had an alkaline pH optimum, required magnesium ions for activity, and degraded native and heat-denatured nucleic acids. M . laidlazvii RNase degraded RNA-core. RNA inhibited the degradation of DNA by M . laidlawii deoxyribonuclease. The implications of these findings with respect to the effects of RNA and DNA on growth of M . laidlazvii are discussed.

Distribution of phagocytic cells in the rat endometrium during the estrous cycle and early gestation was examined by histological, electron microscopic, and histochemical techniques. The results show that numerous macrophages emerge around the nidus shortly after the onset of ovum implantation. Such macrophages, however, were not present within the decidua, suggesting that this tissue may be a protective barrier against the migration of phagocytic cells towards the implants. Approximately 48 hours after the onset of implantation, the number of endometrial macrophages decreased dramatically.