Among the expressed genes associated with the switch-over of Dictyostelium cells from cell proliferation to differentiation, the Calfumirin-1 (CAF1) gene has been shown to be preferentially expressed at the initial step of differentiation, encoding a novel Ca2+-binding protein (Abe & Maeda 1995). To analyze precisely the function of CAF1, transformants overexpressing the CAF1 mRNA at the vegetative growth phase and also CAF1-null mutants were prepared, and their developmental features were compared with those of parental wild-type cells. As a result, the CAF1-overexpression was found to promote cell differentiation, possibly through prompt induction of the cAMP receptor 1 (CAR1) gene expression. In addition, the CAF1-overexpressing cells were able to differentiate even under low external Ca2+ ([Ca2+]e) conditions around 10(-6) mol/L at which non-transformed wild-type cells never differentiated. Unexpectedly, however, the CAF1-null mutant produced by homologous recombination exhibited apparently normal development to form fruiting bodies on non-nutrient agar. These results seem to indicate that CAF1-overexpression has a stimulatory effect on differentiation, but that the CAF1 protein is not necessarily required for the phase-shift of cells from growth to differentiation.