Evidence for heterogeneity of postheparin lipolytic activities in plasma was explored. After low or high doses of heparin, activities were measured in assays with phospholipid or monoglyceride substrate and with triglyceride substrate in assays using either calcium or albumin as the fatty acid acceptor. An excellent correlation between all assays was obtained in normals after both low and high doses of heparin over a wide range of individual response. In selected subjects with hypertriglyceridemia known to be deficient in response to heparin, all responses measured with phospholipid or triglyceride were below the lower limit of normal response. Some responses with monoglyceride substrate were within the normal range, pathophysiological evidence of a separate monoglyceridase. There was no evidence in either group of separate triglyceride lipase and phospholipase activities. The closest correlation between assays in normals was between triglyceride substrate with calcium as the fatty acid acceptor and the assay with monoglyceride. No differential effect upon lipolytic activities was found (1) after preincubation of the plasma, (2) with inhibitors in the assay, or (3) with preincubation of plasma and inhibitors prior to the assay.