To determine whether lipopolysaccharide (LPS) structures of Campylobacter fetus are related to the three known heat-stable serogroups, proteinase K-treated whole cell lysates obtained from strains of each serogroup were electrophoresed in polyacrylamide gels. All strains had smooth-type LPS with multiple high-molecular-weight repeating units. The profiles of serogroup A from C. fetus subsp. fetus and from C. fetus subsp. venerealis were identical, but they were different from those of C. fetus subsp. fetus serogroups B and AB. When we immunoblotted the LPS of these serogroups with normal or immune rabbit serum we found homologous recognition between serogroups A from C. fetus subsp. fetus and C. fetus subsp. venerealis. Similarly, serogroups AB and B from C. fetus subsp. fetus showed homologous recognition. However, antiserum against serogroup A did not recognize serogroups B and AB and vice versa. Absorption studies confirmed the identity of LPS from all serogroup A C. fetus strains and cross-reactivity of the serogroup B and AB strains with one another. Serogroup A strains were resistant to the bactericidal activity in normal human serum, whereas serogroup B and AB strains generally were susceptible; isolates from humans predominantly belonged to serogroup A. Results of these studies suggest that the LPS composition forms the basis for the heat-stable serotyping system for C. fetus and that the structural and antigenic variants are associated with differential serum susceptibility.