Cultured fibroblasts from patients with familial hyperapobetalipoproteinemia (hyperapoB) were used to determine if a defect in lipid metabolism was present. Three basic proteins (BP I, BP II, and BP III) were isolated from normal human serum by preparative isoelectric focusing, preparative SDS/PAGE, and reversed-phase HPLC. The Mr and pI values of these proteins were 14,000 and 9.10 for BP I, 27,500 and 8.48 for BP II, and 55,000 and 8.73 for BP III. These proteins differed significantly in their content of arginine, cysteine, proline, histidine, serine, and methionine. BP I appears to be the same protein as acylation-stimulating protein, but BP II and BP III appeared different from acylation-stimulating protein and other lipid carrier proteins. BP I, BP II, and BP III stimulated the incorporation of [14C]oleate into lipid esters in normal fibroblasts, an effect that was time and concentration dependent. In hyperapoB cells, BP II markedly increased (up to 9-fold) the incorporation of [14C]oleate into cholesteryl ester compared with that in normal cells; in addition, there was a 50% decrease in the stimulation of triglyceride acylation and cholesterol esterification with BP I. No difference between normal and hyperapoB cells was observed with BP III. In summary, the identification of another serum basic protein, BP II, led to the elucidation of another cellular defect in hyperapoB fibroblasts, enhanced cholesterol esterification, which may be related to the precocious atherosclerosis and abnormal lipoprotein metabolism seen in hyperapoB.