TAF (T-cell activating factor), produced by peritoneal macrophages and assayed by its ability to potentiate DNA synthesis in macrophage-depleted lymph node cells stimulated with Phytochemagglutinin-concanavalin A, was shown to contain catheptic carboxypeptidase B (CPB) which accounted almost quantitatively for its potentiating activity. TAF action was inhibited by CPB inhibitors and could be mimicked by commercial pig pancreas CPB. The activity was absorbed from active supernatants on EACA-Sepharose and could be eluted with free EACA (epsilon aminocaproic acid).