The epidemiological associations between the plasma concentrations of several components of high density lipoprotein (HDL) and plasma lecithin:cholesterol acyltransferase (LCAT) concentration have been studied in 101 men aged 52-67 years. Subjects were apparently healthy, and had been selected to provide a wide range of HDL-cholesterol levels. A weak positive correlation was observed between plasma total HDL-cholesterol concentration and LCAT concentration (r = 0.24, P less than 0.02). This reflected an association between HDL3-cholesterol (measured by precipitation) and enzyme concentration (r = 0.21, P less than 0.05). Apoprotein (apo) A-II concentration was also positively correlated with LCAT (r = 0.27, P less than 0.01). HDL2-cholesterol and apo A-I concentration were unrelated to LCAT concentration, as also were the HDL2/HDL3 and HDL-cholesterol/apo A-I ratios. The associations of HDL3 cholesterol and apo A-II with LCAT were strengthened when allowance was made by multiple regression for the effect of log plasma triglyceride; under these circumstances variation in LCAT explained statistically 8% of the variance in HDL3-cholesterol, and 10% of that in apo A-II.