Previous work has shown that specific helper T cells are required for the primary induction of delayed-type hypersensitivity (DTH). Conditions are defined here under which the primary induction by antigen of precursor helper T cells only occurs in the presence of specific, irradiated effector T cells, demonstrating that the induction of helper T cells requires T-T cooperation. The interaction between precursor and effector helper T cells is mediated by the recognition of epitopes that must be physically linked to one another. In more detail, hapten-Ficoll conjugates and xenogeneic red blood cells induce medium-density but not low-density cultures of unprimed murine spleen cells to express antigen-specific DTH. Low-density cultures do not support the induction of DTH unless they are supplemented with specific irradiated helper T cells. These helper T cells are themselves induced when antigen is added to medium-density but not low-density cultures. Precursor helper T cells in low-density cultures are only induced by antigen in the presence of additional specific irradiated T cells. Further experiments were directed at analyzing the nature of this T-T interaction. Irradiated hapten-primed T cells help the induction of precursor helper T cells specific for burro red blood cells (BRBC) in the presence of haptenated BRBC and chicken red blood cells (CRBC), but do not help in the presence of haptenated CRBC and BRBC. These experiments demonstrate that the interaction between precursor and effector T cells is mediated by the linked recognition of antigen. These findings show that the induction of precursor cells for both DTH reactivity, and those T cells able to help in the induction of DTH, require specific helper T cells. It is further shown that the induction of T cells able to help in the induction of helper precursor cells takes place in medium-density but not low-density cultures. In order words, antigen, when added to medium-density cultures of normal spleen cells, induces T cells able to mediate DTH, and T cells able to help in the induction of these helper T cells, whereas antigen induces none of these T cells when added to low-density cultures unless appropriate specific helper T cells are added.(ABSTRACT TRUNCATED AT 400 WORDS)