Phytohemagglutinin (PHA) is generally regarded as a weak immunosuppressive agent. Calne et al. (1) initially reported that it can potentiate the immunosuppressive action of azathioprine in dogs with renal allografts when the two substances were given together. Later on, Gertner et al. (2) recorded again that combination of PHA with either azathioprine or prednisone increased markedly the survival time of skin allografts in dogs and decreased the need for toxic doses of azathioprine and prednisone. Recently, Stefani and Moore (3) suggested that there might be a synergistic effect of PHA and antilymphocyte serum (ALS) in preventing allograft rejection in the clinical situation. In view of the previous evidence that PHA can significantly prolong the survival time of skin xenografts in mice (4) and that antithymocyte serum (ATS) has a greater immunosuppressive effect than ALS on allograft survival (5, 6), it seemed justifiable to do a preliminary experiment concerning skin xenografts in mice treated with a combination of PHA and ATS to see whether a significant synergistic effect could be obtained in this model.