A triacylglycerol (TG) lipase (EC 188.8.131.52), assayed by monitoring [1-14C]-oleic acid release from [carboxyl-14C]-triolein after liquid-liquid partition of the fatty acid from the unhydrolyzed triacylglycerol substrate, was isolated and partially purified from steelhead trout adipose tissue. The TG lipase was resolved from contaminating lipoprotein lipase by heparin-sepharose affinity chromatography and purified ca. 71-fold over the original fraction. Optimal enzyme activity occurred at pH 7.5. The purified enzyme migrated on SDS-polyacrylamide gels with an apparent molecular weight of 48,000.