We have studied cytokine secretion by the human bladder carcinoma cell line 5637 using growth factor-dependent cell line bioassays and specific ELISA. The 5637 cell line-conditioned medium (5637 CM) stimulated proliferation of human growth factor-dependent leukemia cell lines in a dose-dependent fashion. Quantitation by specific ELISA and by bioassays detected high amounts of G-CSF and GM-CSF and smaller quantities of IL-1beta, M-CSF and SCF in 5637 CM; the concentration of IL-3 was below the detection level of the ELISA, if present at all. The G-CSF and GM-CSF activities secreted by 5637 cells could be inhibited specifically by neutralizing anti-G-CSF and anti-GM-CSF antibodies. In conclusion, 5637 bladder carcinoma cells constitutively produce and secrete several functionally active cytokines; 5637 CM is a valuable, reliable and inexpensive source for cytokines, for instance for the culture of growth factor-responsive or -dependent hematopoietic primary cells and cell lines.