Junction nevus, dermal nevus, melanosis circumscripta praecancerosa Dubreuilh, superficial spreading melanoma, and nodular melanoma were investigated and characterized by use of the formalin induced fluorescence method (FIF). In the vicinity of junctional nevus cell clusters and near tumor cells of the superficial spreading melanoma increased numbers of melanocytes are found. These show different types of dendritic branching. Spherical nevus cells however are completely devoid of dendritic processes. On the other hand, the atypical pigment cells in melanosis circumscripta praecancerosa Dubreuilh exhibit a shape similar to that of melanocytes, whereas the globular cells of superficial spreading melanoma have the appearance of nevus cells. The arrangement of nodular melanoma cells resembles that observed in dermal nevus. However the characteristic decrease in fluorescence intensity from epidermal junction to deeper dermis as observed in the dermal nevus was missed in nodular melanomas. Dendritic pigment cells displaying formalin induced fluorescence (FIF) could be demonstrated in all types of malignant melanomas investigated in the present study. The fluorophores of the pigment lesions are characterized microspectrofluorimetrically by (1) ill-defined emission maxima between 470 and 490 nm and (2) a clear-cut excitation maximum at 430 nm accompanied by a lower one at 320 nm. Hydrochloric acid vapor induces a hyposochromic shift of the 430 nm excitation maximum to 370-380 nm and a marked elevation of the 320 nm maximum. These results indicate fluorophores of DOPA and its derivatives; in this respect there are no marked differences between melanocytes, nevus cells and the cells of malignant melanoma.