Dendritic cells (DC) are thought to be distributed throughout lymphoid and most nonlymphoid tissues. Single cell suspensions were prepared from mouse hearts and kidneys. Subsets of MHC class II-positive (Ia+) leukocytes from both sources expressed markers such as CDw32 Fc receptors, F4/80, and complement receptor type 3 (CD11b/CD18). The capacity of these cells to initiate primary in vitro immune responses was assessed using oxidative mitogenesis and allogeneic mixed leukocyte responses. After fractionation by density centrifugation, cell sorting, immunomagnetic bead separation, or cell panning, the stimulatory activity of kidney cell suspensions was found to reside in the low density, Ia+ leukocyte fractions after overnight culture (day 1). In contrast, freshly isolated (day 0) cells had considerably less or no activity in these assays. However, depletion of Ia+ or CD45+ cells on day 0 followed by overnight culture removed the stimulatory activity on day 1. Therefore, day 0 kidney cells contain Ia+ leukocytes that can acquire or up-regulate their stimulatory activity during overnight culture. Similar observations were made for cells isolated from hearts, except that a population of uncharacterized nonleukocytes with stimulatory activity was detected on day 0 but not day 1. The phagocytic capacity of the leukocytes was then examined. Subsets of Ia+ cells phagocytosed zymosan, as shown by two-color flow cytometry and other immunofluorescence studies, and the zymosan-positive cells from kidney were able to initiate primary responses. Overall, these data demonstrate the existence of DC in kidneys and hearts, and suggest that in situ these cells resemble immature rather than mature DC.