Activity of the interferon-induced enzyme 2',5'-oligoadenylate (2-5A) synthetase was measured in peripheral blood mononuclear cells (PBMC) of 32 cancer patients treated with recombinant human gamma-interferon (rHGIF). Pretreatment activities varied widely but remained constant for each individual. The minimum dose of rHGIF that consistently stimulated activity was 12 MU/m2, although in some patients as little as 0.004 MU/m2 was stimulatory. Enzyme activity in responding patients increased threefold at 24 h after the initial infusion and remained elevated nearly twofold at 72 h independent of dose. This increase is less than that observed after infusions of alpha-interferon and parallels results observed in vitro. Pretreatment enzyme activity did not correlate with response. Twenty-minute infusions of a fixed dose were as effective as four- and 24-h infusions in raising enzyme levels. The level of activity achieved after the initial infusion could not be sustained with twice-weekly infusions. We conclude that rHGIF does induce 2-5A synthetase activity in PBMC but not to the levels induced by alpha-interferon. The observation that brief infusions of low doses can be maximally stimulatory in vivo suggests that the response of PBMC in vivo is being confounded by factors yet to be determined. Furthermore, it appears that the capacity of PBMC to maximally respond to rHGIF in vivo, with increases in enzyme activity, is transient.