A microtechnique is described for inducing IL-1 activity in vitro from frozen human blood mononuclear cells (BMNC) using the lymphocyte-activating factor (LAF) method. When compared to the conventional culture system, this micromethod offers several advantages: (1) the complete assay requires a total of 1.0 × 105 mononuclear cells which is easily obtained from 1 ml of whole blood; (2) the method of isolation and freezing of cells preserves their viability and ability to produce IL-1 for several weeks; (3) when compared to the conventional test using fresh BMNC, frozen cells produce the same amount of IL-1. The microtechnique described here is highly reproducible and provides a useful tool for evaluating the production of IL-1 from BMNC of human subjects in clinical studies including those with immunodeficiencies, malnutrition and malignant diseases.