We have evaluated the requirements for non-activated allogeneic B cells to present alloantigen in primary MLC. We found that the generation of an MLR was dependent on the addition of exogenous IL-1 and IL-6 to the cultures; neither lymphokine by itself was capable of significantly stimulating the response. Our results suggest that the concentration of these lymphokines present in a standard MLR culture may serve to regulate the magnitude of the proliferative response. The B cells that co-stimulated with IL-1 and IL-6 were found to be contained almost exclusively within the large, low density population. Presumably, large low density B cells have attained a state in which they are particularly effective at presentation of alloantigen. Together our findings indicate that three components are required for Ag presentation in primary MLC: class II MHC-positive stimulator cells that have attained the appropriate state of differentiation and the presence of both IL-1 and IL-6.