Twelve patients with advanced colorectal carcinoma received oral treatment with RU 41.740, an immunomodulatory drug obtained from Klebsiella pneumoniae. The patients received three courses of RU 41.740, each consisting of 8 mg daily for 7 consecutive days, with free intervals of 3 weeks. This treatment did not significantly change the distribution of various lymphocyte subsets in the blood or the NK activity of the lymphocytes. However, PHA reactivity of purified lymphocytes increased significantly and exhibited a 2-3-fold enhancement 3 weeks after the last course. Such an increase was not observed in lymphocyte preparations which were not depleted of monocytes. It is concluded that 41.740 may be immunopharmacologically active in man when administered by the oral route.