Infantile (juvenile) capillary hemangiomas are vascular neoplasms which can appear quite infiltrative histologically and are characterized by cords of cells with areas of marked cellularity. While vessels can be distinguished in most cases, there are many cells which do not appear to be endothelial in origin. We labeled 5 such cases with antibodies directed against factor VIII-related antigen, CD34, alpha actin, factor XIIIa and PC-10. Anti-factor VIII-related antigen labeled all endothelial cells and did not label cells away from vessels. Anti-CD34 recognized similar cells and also stained a subset of interstitial cells. Anti-alpha muscle defined pericytes and stained few interstitial cells and none of the endothelial cells. Many of the interstitial spindled and cuboidal cells stained strongly with anti-factor XIIIa. The majority of the mitotic activity was concentrated in the interstitial cells. These observations lend support to the concept that infantile (juvenile) capillary hemangioma is a tumor of primitive cells with capabilities for differentiating toward endothelial cells and pericytes. It is not clear whether a similar stem cell population gives rise to dermal dendrocytes, or whether these represent an immune response to the neoplasm.