Six carcinoma cell lines elaborating colony-stimulating factors (CSFs) were examined by enzyme-linked immunosorbent assay and Northern blotting to determine whether or not they co-produced interleukin-1 alpha (IL-1 alpha), IL-1 beta and IL-6. All 6 cell lines were co-producers; IL-1 alpha was produced in all 6, IL-6 in 5 and IL-1 beta in 3 of them. These results indicate that IL-1 and IL-6 are commonly co-produced in CSF-producing tumors.