After contact with human serum, a series of proteins become exposed on the surface membranes of schistosomula of Schistosoma mansoni as revealed by radioiodination of the intact parasites. Among the proteins, a doublet (Mr 45 000) is particularly prominent. These doublet proteins, which are believed to be parasite-derived, become apparent after a very short time of incubation with human serum (10 min or less) and are expressed on the surface membranes after contact with a high molecular weight component of human serum (Mr greater than 80 000). Pretreatment of the parasites with 1.25 mM colchicine or fixation with 2% glutaraldehyde does not prevent the serum-induced expression of the doublet proteins. Extraction of the parasites with chloroform:methanol 2:1 (v/v), however, blocks the human serum effect. Affinity chromatography using immobilized low density lipoproteins (LDL) from human serum shows a tight binding between the 125I-labelled 45 kDa doublet to the LDL. The possible role of the 45 kDa doublet as a receptor for LDL is discussed.