A remarkable aspect of Dictyostelium development is that cells form evenly sized groups of approximately 2 x 10(4) cells. A secreted 450 kDa protein complex called counting factor (CF) regulates the number of cells per group. We find that CF regulates group size by repressing cell-cell adhesion. In both experiments and computer simulations, high levels of CF (and thus low adhesion) result in aggregation streams breaking up into small groups, while no CF (and thus high adhesion) results in no stream breakup and large groups. These results suggest that in Dictyostelium and possibly other systems a secreted factor regulating cell-cell adhesion can regulate the size of a group of cells.