The positive antitumor effect of Ukrain was shown following i.m. injection of a mixture of this drug with Krebs-2 carcinoma cells to CBA mice. The protective effect was suggested to be a result of the influx of macrophages into the site of Ukrain injection. In contrast, contralateral administration of Ukrain had a negative effect, confirming our hypothesis that Ukrain attracts cytolytic macrophages. In the murine HA-1 hepatoma model treated by single or repeated Ukrain injections, a moderate positive effect of Ukrain was related to increased macrophage (alpha-1-proteinase inhibitor (PI) secretion. The role of tumor cells and macrophages in the secretion of PI (the main inhibitor of serine proteinases) during tumor development is discussed.