Properties of adenyl cyclase from Ehrlich ascites tumor cells were studied. The enzyme was stimulated by catecholamines, prostaglandin E1 (but not F1α or F1β), and NaF; it had a pH optimum around 8. Calcium and thiol reagents reduced both basal and stimulated activities. Sonication destroyed activity; storage at 0–4° for 5 h resulted in small losses of NaF and adrenaline-stimulated but not basal activity. Half maximal stimulation was seen at 0.03 μM for L-adrenaline, 0.05 μM for isopropylnoradrenaline, and 5 μM for L-phenylephrine. Propranolol blocked adrenaline and phenylephrine action, and phentolamine had no effect. Stimulatory effects by adrenaline and prostaglandin E1 were not additive. The Km values for ATP were in the range of 0.02–0.15 mM and were not changed by NaF or adrenaline; likewise, Km values for Mg2+ (0.25–0.36 mM) were not affected.