A panel of melanoma cell lines derived from 7 primary and 20 metastatic lesions was tested for the production of interleukin 1 (IL-1) in standard mouse thymocyte costimulation assays. Constitutively produced IL-1 activity was found in the conditioned media of 4 of 7 primary and 5 of 20 metastatic melanoma cell lines tested. Four of 9 cell lines secreting IL-1 were also shown to contain cell-associated activity in their lysates. Melanoma-conditioned media were, however, unable to support the growth of CTLL, an interleukin 2-dependent cell line. The secreted IL-1 activity was significantly inhibited by antibodies to recombinant IL-1 alpha (3 of 3 lines), but not antibody to recombinant IL-1 beta. When conditioned medium from one cell line was fractionated on a Superose 12 column by fast protein liquid chromatography, a major peak of activity eluted at Mr 22,500-27,500. The presence of 2.2-kilobase mRNA hybridizing a probe for IL-1 alpha and 1.6-kilobase mRNA hybridizing a probe for IL-1 beta was detected by Northern blot in 3 of 4 secreting cell lines but not in a nonsecreting line. Taken together, these results suggest that cultured melanoma cells produce the cytokine IL-1 alpha, although the relationship between melanoma IL-1 and monocyte IL-1 is unclear. The production of IL-1 by melanoma cells is of interest because of its potential roles in the biology of melanoma through direct effects on tumor growth or through indirect effects on adjacent stromal and endothelial cells and infiltrating lymphoid cells.