or a non specific inhibitor of cell proliferation the effect on u373 cells was analyzed proliferation of u373 cells is stimulated by il 1 but not by il 6 see fig2 1 u00d7 10 6 hl 60 cells ml were treated with 10 ng ml pma for 24 hours the cells were transferred into rpmi 2 and incubated for 3 days further the supernatant was collected and assayed in the u373 assay no inhibitor of il 1 stimulated proliferation or non specific inhibitor of cell growth was detected in the culture fluids of pma induced hl 60 cells as determined by the u373 assay in fact hl 60 culture fluids were found to stimulate proliferation of u373 cells presumably due to the presence of il 1 in the supernatant control experiments using anti il 1 gave the expected results in addition the hl 60 inhibitor did not inhibit il 2 dependent or non specific proliferation of ctll cells the initial studies were expanded to determine the best conditions for induction of inhibitor optimal inhibitor production was observed using hl 60 cell densities ranging from 0 5 to 2 0 u00d7 10 6 cells ml and pma concentrations from 1 10 ng ml see fig3.