A bioassay for bovine interleukin-1 (IL1) activity is described. The assay is based on the IL1-stimulated proliferation of a mouse T-lymphocyte cell line, D10(N4)M. Bovine mononuclear cells stimulated with lipopolysaccharide produce an interleukin-1-like activity which stimulated the growth of the D10(N4)M cell line in a dose-dependent manner. The stimulatory activity was neutralised by a combination of both anti-human IL1 alpha and anti-human IL1 beta sera. The quantity of IL1-like activity released from the mononuclear cells increased asymptotically with increasing lipopolysaccharide dose.