These observation suggest that dendritic leukocytes from several different non-lymphoid organs in situ are functionally immature and that in this respect they more closely resemble epidermal LC than mature lymphoid DC. The exception appears to be the interstitial dendritic leukocytes from small and large intestinal lamina propria and Peyer's patches, where functional maturation could be attributed to constitutively secreted GM-CSF by lamina propria cell in situ, or alternatively to the isolation procedure which might lead to functional maturation of gut DC. After overnight culture, and possibly following organ transplantation, interstitial dendritic leukocytes may mature into potent activators of antigen-specific T-cell proliferation (immunostimulation). Further studies are needed to characterize dendritic leukocytes in solid non-lymphoid organs, and these may lead to new strategies for overcoming graft rejection by inhibiting the maturation of dendritic leukocytes after transplantation.