Abstract D. discoideum cells contain surface receptors for the chemoattractant cAMP which are composed of fast and slowly dissociating binding sites with half-lifes of respectively about 1 s and 15 s (Van Haastert and De Wit, J. Biol. Chem. 259 , 13321–13328). In membranes prepared by shearing the cells through a Nucleopore filter, ATP has no effect on cAMP-binding at equilibrium, but the number of slowly dissociating sites is increased about 2-fold by ATP while their apparent affinity and off-rate are not altered by ATP. The effect of ATP is stimulated about 3-fold by Ca 2+ with a half maximal effect at 100 μM Ca 2+. The tumor promoting phorbol ester, phorbol 12-myristate 13-acetate (PMA), increases this Ca 2+-sensitivity of the ATP effect to about 0.2 μM Ca 2+. These data suggest that a specific subpopulation of cAMP receptors in membranes from D. discoideum is altered by the action of protein kinase C.