The common cell type involved in the humoral and cellular responses is the T-cell. Functionally the T-cell, active in the humoral response, is known as Thelper (Th), the T-cell active in the cellular response as Tcellular (Tc). The present experiments are used to study the relationship between Tc and Th. IP immunization, with suboptimal doses of antigen, did not result in antibody formation but induced both a DH (equals Tc) and a Th population. Th and Tc activity changed in a similar way with increasing doses of antigen. Spleen cells taken from mice, which were immunized under conditions which only invoke cellular activity, were able to function even in small numbers as Th in transfer experiments. Educated thymus cells and cells obtained from cortisone or CY treated donor mice had an increased Th and Tc activity when transferred into irradiated recipients. Education in the presence of a high antigen dose reduced both activities below control levels. Cross reactivity between HRBC and SRBC in the hormoral response is known to be at the Th level. It was demonstrated that this cross reactivity also exists at the Tc level. Induction of DH with one antigen acted as pre-immunization, for the humoral response, on the cross reactive antigen too. These results suggest a correlation between the T-cells involved in DH and the Th for antibody formation.