Piliated type 1 Neisseria gonorrhoeae attached to 50% of human sperm after incubation of mixtures in vitro for 15 min at 35 C. In contrast, nonpiliated type 4 N. gonorrhoeae attached to only 23% of sperm. Similar results were obtained with three different strains of N. gonorrhoeae. Treatment with heat or formaldehyde to kill bacteria did not affect the amount of attachment by piliated or nonpiliated types. Escherichia coli and N. subflava, other species of piliated bacteria, attached to about 40% of sperm, and the nonpiliated species N. meningitidis and N. catarrhalis attached to a comparable number of sperm, as did type 4 N. gonorrhoeae. Prior incubation of type 1 N. gonorrhoeae with purified antibody prepared against gonococcal pili reduced the percentage of sperm with attached bacteria to the same level as that for nonpiliated type 4 gonococci. Similar treatment of other piliated organisms or of nonpiliated Neisseria did not affect the attachment of the bacteria to sperm.