The authors tested whether human duodenojejunal mucosa is able to synthesize apoproteins from amino acid precursors because lipoprotein-like particles are visualized by electron microscopy in human absorptive cells and because apoproteins are synthesized by the perfused rat intestine. Duodenojejunal biopsies from 21 normal fasting volunteers were incubated with L-[U-14C]leucine; 15.6±3.3 nmoles of the [14C]leucine was incorporated into protein by 100 mg wet weight of biopsies during an incubation of 2 hr. No [14C]leucine was incorporated by boiled biopsies. Homogenates of incubated biopsies were fractionated by ultracentrifugation: 43±2% of the incorporated 14C as found in the d<1.006 fraction; 3.2±0.6%, 8.1±1.0% and 48±2% were found to be associated with the d = 1.006 to 1.063, d = 1.063 to 1.25, and d>1.25 fractions, respectively. The specific activity in the d<1.006 fraction was 5.6 times greater than that in the other fractions. Of the 14C incorporated into the d<1.006 fraction, 10.8, 7.2, and 7.1% were specifically precipitated by rabbit antihuman apoproteins A-I, A-II, and B, respectively. Of the 14C incorporated into the d is 1.006 to 1.063 fraction, 11.3, 4.1 and 8.5% were specifically precipitated by rabbit antihuman apoprotein A-I, A-II, and B, respectively. Approximately 5% of the 14C incorporated by the d = 1.063 to 1.25 fractions were precipitated by rabbit antihuman apoprotein A-I or A-II. None of the d<1.25 fractions precipitated a significant amount of radioactivity with rabbit antihuman apoprotein C-II, or antiarginine-rich apoprotein. None of the antibodies precipitated radioactivity from the d>1.25 fraction. These experiments suggest that human duodenojejunal mucosa is able to synthesize in vitro apoproteins A-I, A-II, and B from amino acid precursors. The specificity of the immunoprecipitates was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis.