Immunofluorescence was used to examine antibodies to cellular antigens in the sera of patients with malignant melanoma. Sera from 60 melanoma patients and from 33 control individuals (13 normal subjects and 20 disease controls) were studied. Ninety % of the melanoma sera were found to have antinuclear antibodies when epithelial cell lines or melanoma lines were used as substrates for their detection, compared to 18% in the control group. Antinucleolar antibodies and anticytoplasmic antibodies were present in 32 and 17%, respectively, in malignant melanoma but none in the controls. Antinuclear and antinucleolar antibodies could be classified into different types according to different patterns of staining and susceptibility of antigens to digestion with DNase, RNase, and trypsin. Certain types of antibodies, such as those showing granular nuclear staining, appeared to be associated with less advanced stages of malignant melanoma, whereas those showing nucleolar and large speckled nuclear staining were associated with more advanced stages of the disease.