The sera of three patients with malignant melanoma showing reactivity with surface antigens of cultured autologous melanoma cells were analyzed by mixed hemadsorption and immune adherence assays in conjunction with absorption tests. In contrast to the melanoma-specific antigens demonstrated previously, the surface antigens detected by these sera occurred on a broad range of nucleated cells, both normal and malignant, from human, monkey, mouse, and chicken sources. Each serum had a characteristic pattern of reactivity in absorption tests, indicating the detection of distinct antigenic systems. Two sera showed auto-, allo-, and xenoreactivity, as well as the capacity to distinguish different cell populations in the same individual. The other serum reacted with an antigen apparently universally present on nucleated cells from a variety of species, but absent on erythrocytes. As these patients had been treated with chemotherapy, this may have played a role in the emergence of these broadly reactive autoantibodies.