AbstractIn this study the antigen‐presenting functions of murine splenic dendritic cells (DC) and macrophages (MΦ) have been compared. DC and MΦ were pulsed with keyhole limpet hemocyanin (KLH) and found to contain approximately 92% and 50% Ia‐positive cells, respectively. The two KLH‐pulsed cell populations had almost equal ability to stimulate KLH‐immune T cells. The antigen‐presenting functions of DC and MΦ were compared by measuring the capacity of KLH‐pulsed, UV‐irradiated cells, which failed to secrete interleukin 1 (IL1), to stimulate KLH‐immune T cells when each culture was supplemented with an equal amount of exogenous IL1. The antigen‐presenting capacity of DC was always greater than that of MΦ. Coculturing of the two kinds of accessory cells upon KLH pulsing resulted in enhanced antigen‐presenting capacity of the mixture relative to the same number of each accessory cell type. That is, collaboration between them in antigen presentation was clearly observed. However, direct contact between DC and MΦ was not obligatory for this collaboration. Furthermore, we found that different types of soluble factors released by each of them might be responsible for the collaboration. In addition, supernatants of concanavalin A‐stimulated spleen cells augmented the antigen‐presenting function of only MΦ. Finally, the antigen‐presenting function of DC and MΦ from athymic mice was almost th