Seven ELISAs were developed by using several combinations of anti-human IL-1β antibodies for detecting interleukin 1β (IL-1β) in cell culture supernatants. These ELISAs have different sensitivities in detecting standard preparations of recombinant human IL-1β (WHO reference standard) compared with conventional preparations of IL-1β produced by stimulated human peripheral blood mononuclear cells. the observed differences were attributed to differences in epitope specificity of the various monoclonal antibodies used and the heterogeneity of IL-1β secreted into culture supernatants. the presence of soluble IL-1 receptor type I did not alter the levels of IL-1β detected by these ELISAs. However, soluble IL-1 receptor type II interfered with the detection of IL-1β to different degrees in these ELISAs. A method involving standarization by means of separate measurement of the amount of receptor and its inhibitory effect in the IL-1β ELISA, yields consistent estimates of the correct IL-1β levels.