The clinical and immunologic effects of a biosynthetic human leukocyte interferon, recombinant leukocyte A interferon (IFL-rA), are reported in eight patients with advanced cancer. Single escalating doses from 3×106 units to 198×106 units were given by intramuscular injection in a phase 1 study. Major toxic effects included pyrexia, fatigue, myalgia, and headache. Data on the effects of IFL-rA on lymphocyte subpopulations and peripheral blood mononuclear-cell surface β2-microglobulin are presented. Four of eight patients had objective tumor regression, indicating that further investigation of this biologically active material is warranted.