Clofibrate, a widely used hypolipidaemic agent was given for twelve weeks to ten subjects with hypertriglyceridaemia. Its effect on lipoprotein-lipids and caeruloplasmin, IgA, IgM, alpha2-microglobulin and transferrin was assessed by comparing analyses at 4, 8 and 12 weeks on therapy with the means of values at two weeks before and at the start of treatment. The normal variation in plasma proteins was assessed in six healthy volunteers during the same period of time. On clofibrate, very low density lipoprotein (VLDL) cholesterol and triglyceride concentrations fell, but the concentrations of cholesterol in low density (LDL) and high density (HDL) lipoproteins showed no consistent change. Caeruloplasmin and IgM concentrations decreased significantly, IgA showed a limited falls (significant only at 8 weeks) and alpha2-macroglobulin did not change. The concentration of transferrin increased on therapy. No relationships were found between the falls in VLDL-lipid concentrations and the alterations in other plasma proteins. No significant variation occurred in the concentrations of lipids or proteins in the normal subjects during the period of study. The results indicate that clofibrate exerts general effects on protein metabolism.