Mouse monoclonal antibody (MoAb) R24 detects the disialoganglioside GD3, a neuroectodermal antigen expressed by normal melanocytes and melanomas. This antibody has been used to define the immunohistochemical distribution of GD3 antigen in normal and malignant human tissues. The present study shows that staining with MoAb R24 in normal organs and tissues where low concentrations of the antigen can be detected. The highest expression of GD3 was detected on the surface of melanoma cells. Therefore, MoAb R24 can be a useful reagent for studying cells of melanocytic lineage, and for imaging and therapeutic studies of melanomas.