Various biological assays are used for qualitative and quantitative measurements of interleukin 1 (IL-1) in supernatants from cell cultures. The purpose of the present study was to compare the specificity and variability of three cellular IL-1 bioassays: the PHA co-stimulatory human T lymphocyte proliferation assay, the PHA co-stimulatory murine thymocyte (THY) proliferation assay, and the 2-step NOB-1 conversion assay. Three different ways of IL-1 unit calculation, based on a semi-logarithmic plot, a double-logarithmic plot, or a probit-analysis plot were also compared. The T lymphocyte assay can be used only to demonstrate qualitative differences in IL-1-like activity, whereas the THY assay is excellent as a semi-quantitative assay, with a low intra-assay variability, but also with a low specificity. The NOB-1 assay is probably more specific with respect to IL-1 measurement, although, with a high intra-assay variance. The THY and the NOB-1 assays both have a high inter-assay variability, and measurement of samples from longitudinal clinical studies must be done in one and the same analysis if quantitative differences are to be illustrated, Probit analysis for unit calculation is recommended. To generate a consensus view as to assay performance, collaborative laboratory studies are needed.