To determine whether IL-1 alpha and/or IL-1 beta protein is expressed by human melanoma tumor in vivo, we first analyzed nine human melanoma cell lines and optimized the in situ detection of these proteins. Three of the melanoma cell lines stained positively for both IL-1 alpha and IL-1 beta using immunohistochemistry (IHC). THe specificity of IHC was confirmed by the ability of purified recombinant IL-1 alpha and IL-1 beta protein to abolish the staining after being adsorbed by their respective antibodies before use in IHC. The three positively staining cell lines were also the only lines to demonstrate IL-1 production by western blot analysis as well as IL-1 secretion by ELISA. Next we examined 29 surgically obtained melanoma tumor specimens (6 primary and 23 metastases) that had been formalin fixed and paraffin embedded. Using the same anti-IL-1 antibodies, 5 of 23 metastatic tumors stained positively. None of the 6 primary lesions stained for either IL-1 alpha or IL-1 beta. Comparison of staining pattern performed on serially sectioned tissue using preimmune serum and antibodies against S-100 protein, melanoma-associated antigen (HMB-45), and CD68 (kappa P1), which recognizes monocyte-macrophage cell lineage, demonstrates for the first time that IL-1 protein is produced by human melanoma tumor cells in vivo. These findings provide the basis for examination of what may be a previously unrecognized biologically distinct subset of patients.