Human blood dendritic cells can be enriched to 40-80% purity by a new technique that is simpler, provides greater yields than prior methods, and resolves other populations that are enriched in monocytes and B and T lymphocytes. The procedure involves separation over two Percoll gradients after 0 and 2 days of culture, followed by removal of contaminating monocytes by panning on plates coated with human Ig. The resultant dendritic cell-enriched fraction is 10 times or more potent than the monocyte-enriched populations in stimulating T-cell proliferative responses to alloantigens and to Con A. Small B lymphocytes are inactive in both systems. Dendritic cells do not initiate mitogenesis to anti-CD3 monoclonal antibodies, a response for which the monocyte appears to be the critical accessory cell.