Patients with autosomal recessive abetalipoproteinemia (ABL) lack in their plasma all lipoproteins containing apolipoprotein (apo)B-100 or B-48. Previous studies have suggested that this is due to the complete absence of apoB. We have investigated whether such patients (n = 10) are able to secrete the lipoprotein(a) (Lp(a] glycoprotein (apo(a] which, in normal plasma, exists as a complex with low density lipoproteins containing apoB-100 (Lp(a) lipoprotein). All 10 patients had reduced but detectable apo(a) levels in plasma (mean, 0.49 mg/dl; range, 0.2-2.03 mg/dl) but no Lp(a) lipoprotein. However, we also detected small amounts (0.2-2.8 mg/dl) of apoB in all patients with ABL. The apoB in the ABL patients had the size of apoB-100 and occurred as a lipid-poor complex with the Lp(a) glycoprotein in a fraction of density 1.22 g/ml. This material may represent partially assembled Lp(a) lipoprotein. There was also uncomplexed apo(a) and apoB-100 in the ABL plasma. The distribution and relative concentration of both proteins in the density fraction greater than 1.06 g/ml varied among patients. The data suggest that in ABL, the assembly of apoB-containing lipoproteins is defective and that apoB-100 may be secreted without its full lipid complement when complexed with apo(a).