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Immunoelectron microscopy was applied to study the antigenic make-up of human and simian immunodeficiency viruses (HIV, SIV) grown in cells expressing either MHC class I (Molt-3) or MHC class I and II (H9) antigens. A variety of antibodies directed against the surface glycopro­ tein gpl20 of HIV and against MHC class I and II antigens were employed. Consistent with earlier observations on the loss of HIV envelope components, gp120 was only weakly demonstra­ ble on the mature virion. MHC class I determinants were present regularly in small amounts on HIV and SIV. Class II antigens, e.g. HLA-DR were found in high density on HIV and SIV grown in H9 cells, but were absent, as expected, on virus grown in Molt-3 cells. These cellular surface antigens are con­ stituents of the virion. The presence of MHC class II antigens in virus preparations used for diagnostic purposes might explain some of the false positive results in HIV serology. Possible biological implications of these virus associated cellular antigens for the pathogenicity of HIV are discussed.

Ten biopsies of oral Kaposi's sarcoma were examined histologically and ultrastructurally. Histologically, early and late tumor stages could be differentiated. On the Ultrastructural level, endothelial‐like and spindle‐shaped tumor cells were revealed. Tumor cells associated with vessel‐like spaces showed partly interrupted basal membranes, Weibel‐Palade bodies, desmosomes and tight‐junctions, while spindle‐shaped cells lacked these Ultrastructural features of endothelial cells. Within the cytoplasm of endothelial‐like cells, aggregates of tubular structures were observed. Histological and Ultrastructural findings in oral Kaposi sarcoma are comparable to those of such other organs as skin and intestines. The Ultrastructural findings indicate an endothelial origin, at least of endothelial‐like tumor cells.

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