Manganese (Mn²⁺) stimulated the progressive motility of human washed sperm in a time- and dose-dependent manner. This confirmed previous data indicating a capacity for activating the adenylyl cyclase of sperm homogenates in vitro. A maintained response was best seen with doses 0.2–1.0 mM. After an initial stimulation, higher concentrations of up to 20 mM were associated with a decline in the response to control levels. Magnesium ions (Mg²⁺) stimulated motility in the same dose range but to less than 50% of the Mn²⁺ response. Other divalent cations such as zinc (Zn²⁺), strontium (Sr²⁺), and Calcium (Ca²⁺) also exerted stimulatory effects to varying degrees in descending order. Metal ion effects on sperm motility may be mediated through a common cation-binding site on the adenylyl cyclase.