Heterologous antisera have been raised in the horse and rabbit against human lymphocytes. Appropriate absorptions on either B or T cells were performed to make antisera specific for human T (anti-HTLA) or B (serum 789) lymphocytes respectively. In addition serum 789 was found to react with circulating monocytes. The percentages of T and B cells detected by anti-HTLA and 789 sera in the different lymphoid organs averaged respectively: 78-7 per cent and 14-7 per cent in peripheral blood, 91-4 per cent and 4-0 per cent in thymus, 73-0 per cent and 14-5 per cent in lymph nodes, 53-6 per cent and 30-0 per cent in spleen, 47-1 per cent and 47-6 per cent in tonsils and 17-5 per cent and 13-5 per cent in bone marrow. Anti-HTLA serum appeared to supress E-rosette formation but did not affect binding of C3-coated erythrocytes. Serum 789 did not prevent E-rosette formation and reduced the number of EAC rosettes by 50 per cent. Anti-HTLA serum was found able to suti-lymphocyte serum, and PWM in the presence of complement; it was found highly mitogenic by itself. Serum 789 decreased the proliferative response to phytomitogens in about the proportion of cells killed by the antiserum. These results indicate that the presence of T cells is necessary for the mitogen-induced proliferation to occur, and that B lymphocytes are induced to proliferate in the presence of T cells and phytomitogens. Anti-HTLA serum was found not to inhibit K-cell activity of lymphocytes against antibody-coated chicken erythrocytes. These antisera appear very useful tools for the study of the role of human B and T lymphocytes involved in in vitro immune reactions.