The effect of low levels of added lymphocytosis-promoting factor (LPF) on the ability of several antigenic preparations isolated from Bordetella pertussis and other bacteria to protect mice against intracerebral infection with B. pertussis was examined. LPF was found to enhance the protective activities of filamentous hemagglutinin, 22S antigen, and fimbriae isolated from B. pertussis. Outer membrane protein preparations from phase I B. pertussis which had LPF removed by haptoglobin affinity columns or inactivated by glutaraldehyde, sodium dodecyl sulfate, or Formalin had reduced protective activities but were made fully protective by the readdition of LPF. Similarly, outer membrane protein preparations from Bordetella bronchiseptica, Bordetella parapertussis, or phase IV B. pertussis lacking LPF were protective only when low levels of LPF were added to the preparations. Outer membrane protein preparations from Neisseria gonorrhoeae or Escherichia coli were nonprotective even in the presence of added LPF. The purified LPF by itself was nonprotective unless treated with glutaraldehyde. LPF that had been detoxified with glutaraldehyde was, however, ineffective at enhancing the protective activity of antigenic preparations. The synergistic effect of LPF is discussed in relation to its known biological properties.