Melanoma associated antigen (MAA) is dispersed in the cytoplasma of melanoma cells, and is more concentrated inside vacuoles and sometimes on the melanosomes. Occasionally the antigen is seen on the cell surface. The antigen is actively shed from living cells. Although the antigen is associated with melanomas, it is not codistributed with the tyrosinase activity associated with melagonesis. The antigen shows codistribution with cathepsin D, which is a marker for lysosomal functions.This antibody NKI/M6 recognizes a high molecular weight proteoglycan with a molecular weight of;450 kD (chondroitin sulfate) and 250 kD (core protein). The antibody NKI/M6 reacts with melanoma cells derived from cell lines and short term cultures and reacts preferentially with melanoma cells in frozen sections. NKI/M6 can also be used to detect melanoma lesions in vivo. NKI/M6 shows cross-reactivity with most naevi and perineurium, and shows weak reactivity with hair follicles.