Antibody responses in three pairs of rabbits inoculated with live Rochalimaea henselae, Rochalimaea quintana and Afipia felis were studied by enzyme immunoassay with whole-cell and bacterial sonicates as antigen. No differences in measured antibody responses were found with the two types of antigen preparation. Two rabbits did not respond with antibody production. In the remaining rabbits there was a low-titred antibody response that showed no significant cross-reaction with related bacteria. After rechallenge the antibody response rose significantly and there was significant cross-reaction with related bacteria. The antigens involved in the antibody response were examined by crossed immunoelectrophoresis. After the initial inoculation 5-7 precipitin lines of the reference diagrams were deflected, including lines which cross-reacted with antigens found in related bacterial species. After reinoculation several more precipitin lines were deflected, including additional lines cross-reacting with antigens present in related bacteria and common bacterial antigens.