The distribution of PAF-acetylhydrolase (PAF-AH) activity in 3 LDL subfractions prepared by density gradient ultracentrifugation as well as the rate of phosphatidylcholine (PC) hydrolysis during oxidation was studied. PAF-AH activity, measured before oxidation, was much higher in LDL3 subfraction (28.4 +/- 8.6 nmol/mg per min) comparing to LDL2 (14.1 +/- 5.8 nmol/mg per min), and to LDL1, 8.7 +/- 3.7 nmol/mg per min. During oxidation, the enzyme activity was continuously decreased and this phenomenon was more pronounced in LDL1. PC hydrolysis was studied measuring the lyso-PC production expressed as lyso-PC/Sph molar ratio. Before oxidation, the lyso-PC/Sph molar ratio, did not differ significantly among the LDL subfractions, whereas, 4 h after the onset of oxidation, it was significantly higher in LDL2 and LDL3 subfractions (0.42 +/- 0.12 and 0.45 +/- 0.10, respectively), comparing to LDL1 (0.29 +/- 0.06). Our results show that the distribution of PAF-AH activity in LDL subfractions is heterogeneous (mainly distributed in LDL2 and LDL3 subfractions) and it is positively correlated with higher lyso-PC production in those subfractions during oxidation. The contribution of this phenomenon to the enhanced susceptibility to oxidation as well as to the higher atherogenicity of the dense LDL subfractions is under investigation.