The immunologic reactivity of some antigens of low passage Borrelia burgdorferi varied with the temperature at which it was cultivated in vitro, with long-term cultivation in vitro, and following passage in white-footed mice, Peromyscus leucopus. The low passaged uncloned strain Sh-2-82 had either more antigens or antigens which were more immunoreactive when grown at various temperatures from 28-39 degrees C. The influence of temperatures was less evident, however with a higher passage of the same strain. Immune sera from white-footed mice experimentally infected with the spirochete were more reactive with antigens of the inoculum than with antigens of spirochetes reisolated later. The antigenicity of the Lyme borreliosis spirochete is becoming more complex and this may relate to the complex life cycle that includes tick and mammalian hosts.