The motility of washed suspensions of human spermatozoa was completely inhibited by tetraphenylboron at concentrations that had little effect on sperm energy metabolism. The inhibition of motility was reversed by quaternary ammonium salts, albumin, caffeine, dibutyryl cyclic AMP and K ions. The addition of ouabain to cells rendered immotile by tetraphenylboron prevented reinitiation of motility by K but not by the other compounds. These observations, together with the effect of tetraphenylboron on the fluorescence of sperm suspensions treated with 1-anilinonaphthalene 8-sulfonic acid, suggest that the binding of tetraphenylboron to sites on the sperm plasma membrane is involved in the inhibition of sperm motility and that cyclic AMP may be involved in the regulation of ion transport across the plasma membrane.