Dependance of adenylate cyclase activity from "aggregation-component" Dictyostelium discoideum cells on ATP concentration has been examined. The enzyme exhibits on-Michaelian kinetics towards its substrate, from which two apparent Km values of 17 micro M and 0.4 mM can be calculated. Several experiments were performed in order to test for a difference between adenylate cyclase activities measured at ATP concentrations corresponding to each Km. Activities were associated with the particulate fraction of the extracts; however attempts have failed to recover them from purified plasma membranes. Loss of activity in cellular extracts, its stimulation by Mn2+ ions, and the increase in basal specific activity during differentiation were identical whether the enzyme was tested at either of the ATP concentrations. Although these experiments do not exclude the possible existence of two distinct molecular entities, they rather favour the hypothesis of an adenylate cyclase exhibiting anti-cooperative kinetics towards ATP.