The effect of supplemental B lymphocytes on the antigen-induced in vitro proliferative response of highly purified lymph node T cells was studied in two immune systems of complete Freund's adjuvant (CFA) and testicular antigen (TA) sensitized guinea pigs. When the T lymphocytes obtained from animals 2 weeks after CFA sensitization, though unresponsive to purified protein derivate of tuberculin (PPD) by themselves, were cultured together with mitomycin C-treated autologous B (Bm) lymphocytes (1:0.5 ratio), they responded well to PPD. The enhancing effect of Bm lymphocytes was less than that of mitomycin C-treated macrophages (M phi m) from the same animals. Analogous helper effect of Bm lymphocytes on the T cell proliferation was observed also in TA-sensitized animals. Additionally, it is noteworthy that the PPD-induced proliferation of T lymphocytes at 2 weeks after CFA sensitization was absolutely dependent on M phi and B lymphocytes, whereas T lymphocytes at 10--14 weeks exhibited a low but significant response by themselves, although their response to PPD was markedly augmented with supplemental M phi m or Bm lymphocytes.