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Guinea-pigs were immunized with emulsions of protein antigens in Freund's complete adjuvant. Draining lymph node cells were cultured for 20 hours together with the immunizing antigen; after harvesting, the culture supernatants were added to fresh lymphocytes from allogeneic or autochthonous guinea-pigs, and these 'recipient' cells were maintained in culture for 3 days. Uptake of [³H]thymidine was measured during the last 18 hours of culture, and revealed that the lymphocyte culture supernatants induced greater thymidine uptake by the 'recipient' lymphocytes than could be expected from the amount of antigen present in the cultures. The soluble factor which mediates this effect was termed 'mitogenic factor'. Analysis of this phenomenon revealed that the mitogenic factor was only generated by interaction between sensitized lymphocytes and the specific antigen used for sensitization; but that once induced, the soluble mitogenic factor was equally active on non-sensitized and sensitized lymphocytes. It is suggested that the mitogenic factor contributes towards the expression of cell-mediated immunity and that it may participate in regulation of the immunological response. [ABSTRACT FROM AUTHOR]