The proliferative response of human peripheral blood mononuclear cells to group A streptococcal vaccine (strain J17A4) was assessed by measurement of [3H]thymidine incorporation. This was maximal after 7 days in cultures containing 1 X 10(5) lymphocytes. Cells from 33 out of 38 adult donors (87%) yielded stimulation indices between 2.0 and 81. Responding cells were predominantly of the OKT4+, Ia+, Tac+ phenotype expressed by activated helper T lymphocytes. In contrast, no response was observed in 10 out of 12 samples of cord blood lymphocytes. Addition of conventional rabbit or monoclonal murine anti-Ia (HLA-DR) antibodies to the culture medium abrogated the vaccine-induced proliferative response. Frequencies of responding cells estimated by limiting dilution analysis were between 1 in 2370 and 1 in 4580 lymphocytes. The results suggest that lymphocyte stimulations induced with streptococcal group A vaccine represented an in vitro anamnestic cellular immune response.