Extracts of various helminthic parasites were studied to determine the molecular carrier of the hapten phosphorylcholine (Pc). Physicochemical studies showed that the carrier was heterogeneous with regard to apparent molecular weight (70,000-greater than 10(6) daltons). Ultracentrifugal analysis of Ascaris suum and Nippostrongylus brasiliensis showed that Pc was associated with low (LDL), high (HDL) and a heterogeneous group of very high density (VHDL) lipoproteins. The majority of Pc was found in the most dense VHDL (d greater than 1.25 g ml-1). The apparent molecular weight and particle diameter of A. suum HDL was 751,000 daltons and 16.2 nm, respectively. The apparent molecular weights and particle diameters of A. suum VHDL ranged from 469,000 to 677,000 daltons and 13.6 to 15.5 nm, respectively. The electrophoretic mobilities of A. suum LDL and HDL were beta and gamma, respectively. A suum VHDL was electrophoretically heterogeneous. It is probable that Pc is associated with the phospholipid phosphatidylcholine.