Two antisera, one raised against activated human T lymphoblasts and one raised against the human T cell line HPB-ALL, have the ability to inhibit antigen-induced T cell proliferation. Antiproliferative activity is absorbed by T blasts and HPB-ALL cells, but not by B lymphoblastoid cell lines, alpha-ethylisothiouronium bromide-treated sheep red blood cell (AET-SRBC) receptor-negative lymphoid cells, or the T cell lines MOLT-4 and HSB. The effects of these antisera do not require the presence of complement and are not attributable to toxicity during culture. The F(ab')2 fragment of anti-T blast antiserum is also active.