Cross-linked dextran (Sephadex) has been reported as an effective inducer of peritoneal exudates resulting in a large yield of macrophages after injection into mice, and macrophages obtained in this manner have been used for a variety of studies. We examined the possibility that cross-linked dextran not only induces peritoneal exudate but functions also as a macrophage activator. Following confirmation of a markedly increased yield of macrophages in Balb/c mice injected with preswollen Sephadex G-25 beads, glass adherence and migration in agarose of these macrophages were compared with the activities of cells isolated following the injection of saline, thioglycollate, or complete Freund's adjuvant (CFA). Dextran administration resulted in a significant increase in these activities in relation to saline and thioglycollate injections, and resembled CFA in the magnitude of its effect. Dextran-induced macrophages were then examined for activation as defined by nonspecific antitumor activity. Cells of a transplantable fibrosarcoma line and a urethane-induced primary lung adenoma served as targets. As compared with macrophages isolated from saline-injected mice, cells induced by Sephadex showed an approximately fivefold and twofold increase in activity against adenoma and fibrosarcoma targets, respectively. To examine the ability of Sephadex to induce macrophage antitumor activity in vitro, peritoneal macrophages were cultured in the presence of the dextran for 24 hours and tested against adenoma targets. The tumoricidal/static activity of such macrophages was significantly increased relative to controls and was nearly identical to the activity of macrophages induced by Sephadex in vivo. It is clear that cross-linked dextran in the form of Sephadex G-25 can function both as an inducer of peritoneal exudate and as an effective macrophage activator with regard to the activation criteria studied.