Recombinant human interferon alpha A/D (IFN alpha A/D) is known to be as active on murine cells as on human cells. We studied the antitumor effect of pure IFN alpha A/D on Meth-A sarcoma subcutaneously transplanted into female syngeneic BALB/c mice. When administered systematically (intraperitoneally), IFN alpha A/D was only marginally (but significantly, P less than 0.05) effective in inhibiting tumor growth. With intralesional injection, however, IFN alpha A/D strongly suppressed the growth of Meth-A sarcoma, even leading to complete tumor regression and to subsequent immunity to Meth-A sarcoma cells in the host mice when the treatment was started early after tumor transplantation and with a high IFN alpha A/D dose. We also found that treatment of mice with IFN alpha A/D increased the level of serum alpha 1-acid glycoprotein, one of the acute-phase proteins.