The IL-1 alpha induced up-regulation of HLA class I and HLA class II (DR) antigen expression on the cell surface of the human breast cancer cell line ZR-75-1 was demonstrated. This was associated with a concomitant increase in TNF alpha production. Coincubation with an anti-TNF alpha neutralizing antibody partially inhibited the IL-1 alpha induced up-regulation of HLA DR antigen but had no effect on IL-1 alpha induced HLA class I up-regulation. These data indicate that IL-1 alpha induced HLA class II (DR) antigen up-regulation in ZR-75-1 cells is partially mediated by TNF alpha and that IL-1 alpha induced HLA class I and class II (DR) antigen up-regulation in ZR-75-1 human breast cancer cells in vitro are mediated by different mechanisms.