Gonococcal infections of relatively long duration were produced in guinea pigs with minimal infective doses ranging from 10(1) to 10(2) colony-forming units. After spontaneous eradication and upon rechallenge with >/=10(8) gonococcal colony-forming units, guinea pigs were refractory to infection. Serum from these guinea pigs was bactericidal in vitro and protected virgin guinea pigs from in vivo challenge. The 7S peak, but not the 19S peak, from gel filtered immune serum demonstrated both bactericidal and passive protective properties. In vitro bactericidal activity of whole immune serum and its 7S peak was abolished after heating at 56 C for 30 min. The in vivo protective ability of the 7S peak was not abolished after heating.