The effect of gemfibrozil treatment (900 mg/day) on serum levels of total cholesterol, HDL-cholesterol, triglyceride, apoproteins A-I, A-II and B as well as HMG-CoA reductase in mononuclear cells was studied in patients with hyperlipoproteinemia types IIa and IIb. After 4 weeks of treatment gemfibrozil reduced total serum cholesterol (IIa: -17%, IIb: -26%), triglyceride (IIa: -39%, IIb: -47%) and apoprotein B (IIa: -22%, IIb: -15%). HDL cholesterol was increased by 20-22% and apoproteins A-I and A-II by 4-11%. Concomitantly, HMG-CoA reductase activity in freshly isolated mononuclear cells was suppressed by 78% in type IIa and 51% in type IIb patients. Continuation of treatment for up to 16 weeks prompted a further decline to 8 and 5% of the initial values, respectively. However, gemfibrozil failed to affect HMG-CoA reductase directly in homogenized or cultured mononuclear cells and did not further promote the suppressive action of LDL when added to the culture medium. Similarly, preincubation with the drug did not significantly modulate the binding or degradation of LDL in the cultured cells. However, LDL from patients with hyperlipoproteinemia types IIa and IIb exhibited enhanced binding and more potent HMG-CoA reductase suppression when isolated after compared to before gemfibrozil treatment. It is suggested that the HMG-CoA reductase inhibition observed in mononuclear cells during gemfibrozil treatment is due to changes in LDL structure affecting LDL receptor binding rather than direct effects of the drug on cellular cholesterol metabolism.