Spleen cells of mice receiving an i.p. injection of a sublethal dose of cyclophosphamide (CY) have the ability to inhibit tumor growth in vitro. This effect is dose dependent and is maximal at the peak of the spleen regeneration which follows the phase of atrophy due to CY toxicity. This cytostasis is neither tumor-specific nor strain-restricted and the cells responsible for this inhibition of tumor cell multiplication have the characteristics of macrophages: they lack the Thy 1-2 antigenic marker, appear in CY treated nude mice, stick on plastic vessels, and are retained by adherence columns (nylon wool or Sephadex G10); their activity is greatly reduced when a specific macrohpage toxic reagent such as carrageenan is added to the cultures. The effector cells are similar to those which are able to suppress the response of normal splenocytes to T and B mitogens, and which appear in the same conditions of induction, after CY injection.