The dye-uptake method (A375 assay) for quantitative measurement of biological activity or recombinant human interleukin-1 (IL-1) was established based on the cytostatic activity of IL-1 on human melanoma A375 cells. Although various factors, e.g., incubation temperature, inoculum cell size and cell growth phase, were found to influence the titration to some degree, the assay-to-assay fluctuation in the observed titers determined by this method could be made minimum after normalization with the observed titer of a standard IL-1. A good correlation was observed between A375 and LAF assays. Thus, A375 assay was shown to be a stable and reliable method and could be replaced with LAF assay for quantitative measurement of recombinant human IL-1.