A human helper T cell clone (d4), which showed its helper effect on the differentiation of both T and B cells, was established by MLC reaction of normal T cells against a B lymphoblastoid cell line (CESS) followed by cloning in the presence of IL2 and x-irradiated CESS and autologous non-T cells. d4 cells helped the induction of cytotoxic T cells against UV-treated CESS cells. Antigen-stimulated d4 cells secreted helper factor(s) involved in the induction of cytotoxic T cells (killer helper factor(s), KHF), and KHF activity could be separated into two fractions, one with the m.w. of 15,000 to 20,000 and the other with the m.w. of 45,000 to 50,000. The factor with 15,000 to 20,000 m.w. showed IL 2 activity; the other factor showed gamma-interferon activity without IL 2 activity, suggesting that both IL 2 and gamma-interferon exerted KHF activity. d4 cells or their culture supernatant showed helper activity in the induction of IgG in a B cell line (CESS). The helper activity of the supernatant (TRF) was absorbed with CESS cells but not with IL 2-dependent CTLL, whereas KHF activity was absorbed with IL 2-dependent CTLL but not with CESS cells. The results showed that TRF and KHF were distinct molecules and a single helper T cell clone could secrete helper factors for both B and T cells.