Alpha-factor-mediated cell cycle arrest and mating inhibition of a mating-type cells of Saccharomyces cerevisiae have been examined in liquid cultures. Cell cycle arrest may be monitored unambiguously by the appearance of morphologically abnormal cells after administration of alpha factor, whereas mating inhibition is determined by comparing the mating efficiency in the absence or presence of added alpha factor. For both cell cycle arrest and mating inhibition, a dose-dependent response may be observed at limiting concentrations of the pheromone. If cell cycle arrest and mating inhibition require a small number of alpha-factor molecules, one might expect that responsive/nonresponsive cells = K(alpha factor)(N) where N is the order of dependence of cell cycle arrest (or mating inhibition) on alpha-factor concentration. The value of N has been determined to be 0.98 +/- 0.18 (standard error of the mean) for cell cycle arrest and 1.08 +/- 0.32 for mating inhibition. These results support the notion that saturation of a single site by alpha factor is sufficient to cause cell cycle arrest or mating inhibition of a mating-type cells.