A rabbit polyclonal antiserum was raised against a synthetic peptide, termed CD2-300, comprizing 18 amino acid resides which are conserved among the cytoplasmic domains of the human, rat and mouse CD2 antigens. Cross-depletion experiments showed that the CD2 monoclonal antibody OKT11 and purified CD2-300 antibodies (Ab) precipitated the same molecules from the surface of human T lymphoblasts. The results of immunoprecipitation analyses indicated that the purified CD2-300 Ab were specific for human and mouse CD2, and that the CD2-300 peptide competitively and specifically inhibited precipitation by CD2-300 Ab in both species. When employed to stain murine tissues, the CD2-300 Ab gave the anticipated pattern of distribution for the CD2 antigen, although there was some nonspecific labeling of non-T cells.