Since the apolipoproteins C transfer to triglyceride-rich particles when they enter the lymph-blood compartment, and since the apo CII/CIII ratio may influence the removal of triglyceride-rich particles from that compartment, the apo CII/CIII ratio of proteins that transfer from plasma to an artificial fat emulsion was studied in hypertriglyceridemic and control patients. It appeared (1) that the apo CII/CIII ratio was lower in 10 patients with primary hypertriglyceridemia (0.06 +/- 0.20) than in 13 normals (1.55 +/- 0.73, p less than 0.001) and (2) that it was lower in 16 patients who previously had secondary hypertriglyceridemia, but who were normotriglyceridemic when tested (0.58 +/- 0.24), than in 15 patients with comparable disease but without secondary hypertriglyceridemia (0.91 +/- 0.47, p less than 0.02). These findings suggest that a low CII/CIII ratio of transferable apolipoprotein contributes to the potential development of secondary hypertriglyceridemia.