Rabbit anti-human beta 2-microglobulin (anti-beta 2m) increased the highest DNA synthesis in unseparated lymphocytes or artificially composed mixtures enriched in T and B cells. In enriched T and B cells no or low stimulation was seen. The maximal response by IgG anti-beta 2m was seen in proportions of enriched T/B cells, being 3:1 for blood lymphocytes and 1:1 for spleen cells, which are the same as the physiological proportions of T and B cells in these lymphoid organs. Whereas unseparated lymphocytes gave a peak response on day 3, enriched B cells had a peak response on day 6. Fab anti-beta 2m did not activate enriched T cells but increased DNA synthesis in enriched B cells to about the same extent as unseparated lymphocytes and mixtures of enriched T and B cells. The proportion of sheep erythrocyte rosette-forming cells (E-RFC) decreased after stimulation by anti-beta 2m and increased after stimulation by phytohaemagglutinin. However, as revealed by autoradiography, a proportion of lymphocytes activated by anti-beta 2m were E-RFC and this proportion increased with increasing stimulation by anti-beta 2m. DNA synthesis induced by anti-beta 2m was unchanged for spleen cells and slightly decreased for blood lymphocytes when phagocytic cells were removed by iron treatment. Supernatants from lymphocytes activated by anti-beta 2m only induced low DNA synthesis in enriched T or B cells. Experiments with mitomycin treated cells indicate that cooperation and close contact between T and B cells are needed for activation by IgG anti-beta 2m. T cells are needed for B cell activation by anti-beta 2m and B cells are required for T cell activation to occur.