THE ACTIVITY of adenylcyclase [l] and the concentration of cyclic AMP [2] become enhanced in untransformed cells as the cell population density increases, but are decreased in transformed cells [3,4]. Moreover, the addition of cyclic AMP or dibutyryl cyclic AMP to the culture medium of transformed or malignant cell cultures can inhibit cell division [5, 6, 7, 81 and restore morphological characteristics [9] of untransformed cells. These observations in vitro suggest that densitydependant cessation of growth is controlled by intracellular cyclic AMP levels and that in some malignant cells this mechanism is defective. However, little work [lo] has been done to test whether cyclic AMP is effective as a carcinostatic agent in vivo. We report the effect of administration of cyclic AMP to mice inoculated with Ehrlich tumour cells. Young adult female A strain mice were injected with ten million Ehrlich ascites tumour cells, either subcutaneously between the scapulae to produce a solid tumour, or intraperitoneally. Three days later drugs were given intraperitoneally and continued twice daily for 4-5 days. One group of animals received cyclic AMP, 10 mg/kg body weight, immediately preceded by aminophylline, 50 mg/kg each in 0.1 ml of saline (test group). A second series of mice received the same dose of aminophylline and O-1 ml of saline (aminophylline group), while other mice received O-1 ml of saline only (control group). After 8 days growth, the subcutaneous tumours were solid, white structures without cysts but with a well-established blood supply.