To understand the local immune events which occur when a novel antigen is encountered in the gut it is necessary to know whether cells from the mucosal tissues are capable of initiating T-cell reactivity. We have examined the capacity of cells isolated from the Peyer's patches and the lamina propria of the murine small intestine to present keyhole limpet haemocyanin (KLH) to naive syngeneic splenic T cells in vitro. The properties of the gut antigen-presenting cells (APC) were compared with those of cells from the spleen and the mesenteric lymph nodes. Results clearly demonstrate that cells from the lamina propria as well as from the Peyer's patches, mesenteric lymph node and spleen can present KLH to naive T cells, inducing strong proliferative reactions comparable in magnitude and kinetics. All the APC populations tested induced interleukin-2 (IL-2) production in primary cultures, although minor differences were noted when lamina propria cells were used as APC. IL-4 was not detected in supernatants from cultures of non-immune T cells in the presence of APC from any tissue. Phenotypic analysis of the cells in cultures of naive T cells, with antigen and APC from different gut-associated tissues revealed important differences. Cells from Peyer's patch, mesenteric lymph node and spleen gave rise to cultures containing largely CD4+CD8- cells. However, cultures in which lamina propria cells acted as APC consisted primarily of CD4-CD8+ cells.