Experiments carried out with the aid of cellophane membranes demonstrate that the morphogenetic block of certain nonaggregating, "aggregateless," mutants may be overcome by diffusible factors excreted by aggregating wild-type cells. The same differentiation process into aggregation-competent cell is observed if mutant amoebae are subjected to external 3':5'-cAMP pulses imposed at 5 min intervals. Wild-type amoebae also respond to cAMP pulses, since the onset of differentiation is more precocious in pulsed than in unpulsed populations. These data suggest that chemotactic signals act as an inducer of cell differentiation.