Recombinant murine interleukin 1 (rIL 1) inhibits 3T3-L1 cell expression of lipoprotein lipase (LPL) activity when present in exceedingly dilute concentration (less than 10(-15) M). The extreme sensitivity of the adipocyte system to rIL 1 far exceeds that of the standard lymphocyte-activating factor assay. However, enzyme suppression is incomplete; even at micromolar concentrations, rIL 1 causes only about a 50% reduction in LPL activity. By contrast, cachectin (tumor necrosis factor) achieves nearly complete LPL suppression at subnanomolar concentrations. Concentrated solutions of rIL 1 are incapable of competing with radiolabeled cachectin for binding sites on 3T3-L1 cells. rIL 1-induced LPL suppression is abolished by the addition of a specific IL 1 neutralizing antiserum to the assay system. rIL 1 appears capable of influencing adipocyte expression of LPL, but apparently acts through a different mechanism than cachectin/TNF.