A rabbit antiserum to the mouse CD8 antigen (Lyt-2/3) was obtained through the use of a synthetic peptide corresponding to the N-terminal segment of the 37 kDa subunit of the CD8 molecular complex. This anti-peptide antiserum detected specifically the membrane antigen from detergent extracts of surface-labeled mouse thymocytes. The efficiency of the immunoprecipitation increased significantly upon treatment of the cell lysates with 0.1% SDS at 100 degrees C before immunopurification. Finally both the 37 kDa and the 32 kDa polypeptides expressing the Lyt-2 antigen were revealed by immunoblotting.