In vivo kinetics of apoB were performed in three apoB-67/apoB-100 heterozygotes using a primed-constant infusion of (5,5,5-(2)H3)-leucine. Mean plasma VLDL and LDL apoB-67 concentrations were 0.05 +/- 0.01 mg/dl and 0.62 +/- 0.17 mg/dl, respectively, which were 0.2% and 2.8% of total plasma apoB concentrations. When cation exchange chromatography was used to separate plasma amino acids from fragments of polyacrylamide gels, the tracer/tracee ratio at plateau for VLDL apoB-67 was less than 50% of that observed when centrifugation was used. Mean fractional catabolic rate for LDL apoB-67 was 2-fold higher when the lower plateau was used in multicompartmental analysis compared to the higher plateau (0.70 +/- 0.21 versus 0.37 +/- 0.06 pools per day, P = 0.06). The lower plateau resulted from introduction of unlabeled leucine during cation exchange chromatography; therefore, all samples were processed with centrifugation. Mean fractional catabolic rates for VLDL and LDL apoB-67 were not significantly different from VLDL and LDL apoB-100 (VLDL: 9.7 +/- 3.4 versus 18.1 +/- 8.6 pools per day, respectively, P = 0.19; LDL: 0.37 +/- 0.06 versus 0.34 +/- 0.11 pools per day, respectively, P = 0.66). Mean secretion rate of VLDL apoB-67 was 5.6% of VLDL apoB-100 (0.20 +/- 0.04 versus 3.6 +/- 1.3 mg/kg/day, P = 0.01). Fifty-three % of apoB-67 was directly removed from VLDL compared to only 3.5% of apoB-100 (P = 0.003), thus accounting for the lower proportion of apoB-67 in LDL (3.3 +/- 1.8%) as compared to VLDL (11.2 +/- 2.5%). Mean LDL production rate for apoB-67 was 2.6% of LDL apoB-100 (0.09 +/- 0.02 versus 3.50 +/- 1.39 mg/kg/day, P = 0.05). Thus, decreased secretion of apoB-67 is responsible for low levels of apoB-67.