This study was designed to quantify the levels of apo(a) and assess their distribution among lipoprotein fractions in type IV hypertriglyceridemia. Plasma density < 1.006 g/mL fraction (VLDL) was obtained by preparative and zonal ultracentrifugation. Apo(a) was detected by immunoblotting with anti-apo(a) antibodies of agarose gel electrophoresed lipoproteins. Apo(a) was consistently found in VLDL in 30 hypertriglyceridemic subjects, but not in 10 normolipidemic or 10 hypercholesterolemic subjects with normal triglyceride levels. Apo(a)B particle concentrations were then measured using a selective 'sandwich' ELISA with anti-apo(a) as the capture antibody and anti-apoB as the detecting antibody. The mean apo(a)B level of VLDL in six hypertriglyceridemic patients was 30% (range 1.3-50%) of the total plasma concentration. Apo(a)-containing particles of both plasma, d < 1.006 g/mL and d > 1.006 g/mL, had the same pre-beta mobility on agarose gel electrophoresis and a similar apparent molecular weight on non-denaturing 2-16% gradient polyacrylamide gel electrophoresis. Fractionation of plasma by zonal ultracentrifugation confirmed the presence of a heterogeneous distribution of apo(a) in hypertriglyceridemic subjects. Apo(a) was present throughout the density range from VLDL to Lp(a). After normalization of plasma triglyceride levels with dietary fish oil, apo(a) was no longer detected in VLDL suggesting that detection of apo(a) in the plasma, d < 1.006 g/mL, density fraction is related to type IV hypertriglyceridemia.