Precipitin lines formed between serum from a goat infected with caprine arthritis-encephalitis virus (CAEV) and radiolabelled viral proteins in polyethylene glycol-concentrated culture medium were excised from immunodiffusion (ID) plates and analysed by polyacrylamide gel electrophoresis. The two major precipitin lines contained the 135 000 mol. wt. glycoprotein (gp 135) and the internal 28 000 mol. wt. structural protein (p28). This method obviates the use of purified proteins or monospecific antisera to positively determine viral constituents in ID precipitin lines formed between a crude antigen preparation and antiserum against whole virus.