The conversion of pig high-density lipoproteins (HDL) (mainly HDL3) to fractions of lower densities was studied by incubating pig plasma for 24 h at 37 degrees C in the presence and absence of lipoprotein lipase from bovine milk, lecithin:cholesterol acyltransferase, cholesteryl ester transfer protein and triacylglycerol-rich particles (very-low-density lipoproteins (VLDL) or Intralipid). The results can be summarized as follows. In the presence of lipoprotein lipase and at a VLDL/HDL mass ratio of 2, the F-1.210 of pig HDL was shifted from 3.3 to 4.2, which is characteristic for human HDL2. This shift was caused by the excessive increase in the free fatty acid content in HDL. If 50 g/l of bovine serum albumin were added prior to incubation, the flotation rate of HDL remained in the HDL2a region. If lecithin:cholesterol acyltransferase was active in fasting pig plasma during incubation, we observed only a negligible increase of F-1.210 in HDL. If pig lipoproteins were incubated with human lipoprotein-free serum as a source of cholesteryl ester transfer activity, a slight increase in the flotation rate of HDL was observed, which was amplified in the presence of active lecithin:cholesterol acyltransferase. Pig HDL was converted to a fraction with F-1.210 of 4.2, which is typical for human HDL2, only if active lecithin:cholesterol acyltransferase, cholesteryl ester transfer protein and triacylglycerol-rich particles were present in the incubation mixture. From our results we also concluded that apolipoprotein A-II plays no role in the HDL2 formation.