Pyran copolymer (NSC 46015) was evaluated with respect to its effect on the rejection of a murine leukemic allograft by BALB/c × DBA/2 F1 (CD2F1) mice. Significant prolongation of allograft survival with production of progressively growing lethal tumors was found following pyran administration. This phenomenon occurred at nontoxic doses of the drug and appeared to be closely related to the timing of pyran injection. Nonspecifically stimulated lymphocyte blast transformation by concanavalin A was not impaired by pyran when lymphocytes were exposed in vitro to the drug. The mechanism of tumor allograft enhancement remains obscure but may be related to allograft size at the time of pyran administration.