A sequential mouse cell culture system is described for the induction and assay of T-helper cells. Unprimed, cortisone-resistant, nylon wool-purified thymocytes cultured with adherent peritoneal exudate cells can be primed in vitro with soluble carrier protein to generate carrier-reactive helper cells. These cultured cells enhance the anti-hapten plaque-forming response of hapten-primed spleen cell cultures to hapten carrier conjugates. The culture conditions, cellular manipulations, and antigen requirements for the optimal induction of helper cells with these purified cell populations is presented. The active helper cell generated in this culture system is a thymus-derived cell which requires macrophages for its induction and must be proliferate in vitro before the manifestation of helper-cell function. Helper cells generated in vitro stimulate both carrier-specific and nonspecific enhancement of splenic anti-hapten responses. The carrier-specific and nonspecific enhancement can be distinguished by the requirement for antigen in the helper cell and spleen cell cultures, the dose of helper cells added to the spleen cell cultures, and by the requirement for additional splenic adherent accessory cell interactions.