Amoebae of the cellular slime mould Dictyostelium discoideum showed stimulated mitogenic activity when exposed to 200μM isoproterenol, an activator of adenyl cyclase, for 30min. Approximately 40% increase in cell proliferation was found at 48h after isoproterenol treatment. A faster and larger plaque formation as well as higher uptake of FITC-labelled E. coli indicates greater phagocytotic activity in the treated cells. A concurrent increase in DNA and protein syntheses was also recorded in the treated cells. Administration of 400μM caffeine or 200μM (+) propranolol brought down the isoproterenol-induced elevation in the cell division rate to control levels. These results are discussed in relation to a precocious activation of adenyl cyclase in the treated cells leading to a transient but significant increase in cell division in this organism.