Cells cultured from different tissues and organs of various aged rats were tested by the mixed hemagglutination assay (MHA) for the presence of several histocompatibility antigens. Chronological or histological variations in expression of antigens determined by the major histocompatibility locus of the rat, Ag-B, were exhibited by cell lines derived from both embryonic and adult tissues. Ag-B antigens were detected on primary cultures of cells from neonatal brain, skin, liver, heart, eye, spleen, and kidney. A peculiarity of the kidney-derived cultures was the presence of patches of epithelial cells which did not react with any antiserum tested, including a heterologous rabbit antirat serum. Non-Ag-B antigens, including a male-associated antigen, were also detected with MHA. These antigens were present on cultured tissue cell lines, but not on any of the organ-derived cultures, except skin. The epithelial-like cells of skin cultures, but not the fibroblasts, were reactive. A collagen-mediated masking phenomenon which markedly reduced or climinated reactivity of non-Ag-B antisera was also revealed.