The role of purified apolipoproteins A-I, A-II and C-I in the process of cholesterol efflux from cultured vascular endothelial and smooth muscle cells was studied. [3H]Cholesterol pre-labelled cultures were exposed to serum-free medium supplemented with free apolipoproteins or with apolipoproteins and phosphatidylcholine liposomes and the cholesterol efflux from the cells was determined. Free apolipoproteins A-I and A-II supported cholesterol efflux from vascular endothelial and smooth muscle cells to a higher extent than apolipoprotein C-I. The ability of free apolipoproteins A-I and A-II to support cholesterol efflux was in correlation with their specific binding to the cultures, while no specific binding of apolipoprotein C-I was detected. The association of apolipoprotein A-I with phosphatidylcholine liposomes resulted in a more than two-fold increase in cholesterol efflux compared to free apolipoprotein A-I, while association of apolipoproteins A-II and C-I with phosphatidylcholine liposomes resulted in a very limited increase in cholesterol efflux above that achieved by the free apolipoproteins. These results suggest that apolipoprotein A-I is involved in cholesterol efflux performed by high density lipoprotein. Furthermore, free apolipoproteins A-I and A-II, but not apolipoprotein C-I may take an active part in cholesterol efflux from endothelial and smooth muscle cells.