1. Lipoprotein lipase was separated from normal human post-heparin plasma by affinity chromatography and assayed with a 14 C-labelled triolein emulsion. No enzyme activity was detected unless whole serum was included in the assay as a source of cofactor, apolipoprotein C-II. 2. After a 10 h fast, serum obtained from 46 normal subjects, eight patients with hypertriglyceridaemia but normal renal function, patients with chronic renal failure (24 undialysed, 20 haemodialysed) and 14 recipients of renal allografts, was added to incubation medium for the assay of lipoprotein lipase to determine the maximum activation of the enzyme. 3. When serum was obtained from normal subjects, maximum activation of the enzyme correlated positively with the concentration of triacylglycerol in the sample. Neither sex nor age had a significant effect on the maximum activation achieved by serum from control subjects. 4. The maximum lipoprotein lipase-activating capacity of serum from uraemic and transplant patients was significantly reduced when compared with serum from healthy controls or from the non-uraemic hypertriglyceridaemic patients. 5. Maximum enzyme activation correlated positively with high-density lipoprotein cholesterol in serum from undialysed patients, but did not correlate positively with total serum triacylglycerols in any of the patient groups. Only in transplant recipients was there a significant inverse relationship between serum creatinine concentrations and maximum enzyme activation. 6. Although lipoprotein lipase activation was impaired in uraemic subjects and renal transplant recipients, this appeared to be due more to the presence of an inhibitor than to cofactor deficiency.