Lymphocytes (A) sensitized in vitro by cells from a HLA-identical sibling (B) for 8 days showed inhibiting effects when added to fresh mixed lymphocyte cultures (MLC) where A responders were stimulated by cells from other family members in a ratio of 1:1:1. In 23 of 31 such pairs tested in 15 families, proliferative activities in these 6-day second-step MLC were inhibited by 54 +/- 18% in the presence of A'B sensitized cells as compared to control cultures with modulating A' cells similarly preincubated but in the absence of B stimulators. In addition, A'B could also suppress MLC responses of B in 12 of the 17 pairs in which this was tested. Inhibition was not due to cytotoxic elimination of stimulators and it was radiation sensitive. Suppression appeared to be specific but it did not seem to be restricted by HLA-A, -B, or -DR determinants. Hence, these results indicate that suppressor cells generated after priming by HLA-identical cells can regulate allogeneic proliferative responses even when they are directed to HLA differences.