Dendritic cells (DC) acquire antigens in peripheral tissues, transport them to lymph nodes and present peptides to T cells. DC are particularly good activators of resting T cells. Murine Langerhans' cells (LC) are efficient at endocytosing and processing antigens but are very weak immunostimulators. In culture LC lose the ability to process antigen and become potent immunostimulators. Other peripheral DC are not well characterized and it is not known if they are similarly weak immunostimulators. We isolated DC from rat Peyer's patches (PP) and lamina propria (LP) of the small intestine, from intestinal lymph (LDC) and mesenteric lymph nodes, and examined their ability to stimulate an allogeneic mixed leucocyte reaction (MLR). Freshly isolated LP DC and PP DC could stimulate a moderate MLR but fresh LDC were significantly more potent. After overnight culture, LDC did not change their potency but DC from LP and PP became as potent as LDC. In contrast, fresh lymph node DC stimulated a MLR or oxidative mitogenesis as efficiently as LDC. These results show that the weak immunostimulation of murine LC is not characteristic of all peripheral DC. We compared the phenotypes of DC from different sites before and after culture. Different populations of DC show marked phenotypic heterogeneity in the expression of surface markers, particularly Thy-1, CD2 and the iC3b receptor. PP and LP DC were similar to MLN DC in their expression of markers, but differed from LDC. After culture there were marked changes in DC surface marker expression and the differences between the populations were reduced. These observations suggest that the heterogeneity observed in fresh populations does not signify different stages of maturation but may represent activation.