In order to quantitate the contribution of triglyceride-rich lipoprotein (TRL) apolipoprotein(a) to total plasma apo(a) concentration in the fed and fasted states, we have studied a group of 20 male subjects (age 49 +/- 3 years) with fasting apo(a) concentrations varying from 39 to 1385 U/l. After a 12-h overnight fast, each subject was given a fat-rich meal (1 g fat/kg body weight) and venous blood samples were obtained at hourly intervals for 10 h. TRL were isolated from bihourly plasma samples by ultracentrifugation (d less than 1.006 g/ml) and apo(a) was measured by radioimmunoassay. Total plasma apo(a) concentration did not change after the meal. However, TRL apo(a) increased significantly (0 h: 3 +/- 1, 4 h: 30 +/- 7 U/l; p less than 0.001) and 'd greater than 1.006' apo(a) decreased (0 h: 267 +/- 56, 4 h: 231 +/- 50 U/l; P less than 0.05). Similar postprandial changes were observed in apoB concentration (TRL apo B at 0 h: 10.3 +/- 1.5, 4 h: 13.6 +/- 1.7 g/l, P less than 0.001, 'd greater than 1.006' apoB at 0 h: 118 +/- 7, 4 h: 110 +/- 7 g/l, P less than 0.001). In the fasted state 2.0 +/- 1.0% and in the fed state (4 h postprandially) 16.0 +/- 4.6% of total plasma apo(a) was found in the TRL fraction. Eleven subjects had less than 10% of total apo(a) in TRL, 5 had 25% or more apo(a) in TRL in the fed state. Postprandial increase in TRL apo(a) was significantly correlated (r = 0.75, P less than 0.001) with increase in plasma triglycerides. TRL apo(a) concentration in the fed state was not correlated with total fasting cholesterol, triglyceride, apo(a) or HDL cholesterol concentration. We conclude that in some individuals, TRL apo(a) makes a significant contribution to total plasma apo(a) concentration in the fed state.