A middle-aged alcoholic man had low C3 levels during two consecutive episodes of bacteremia with Campylobacter fetus, with a return of C3 levels to normal during the interbacteremic period. Total complement levels remained below normal throughout the patient's illness, whereas C4 and C3 activator values were in the normal or supranormal range. A rise and fall in hemagglutinating antibody titers, documenting an immune response, coupled with reasonable antimicrobial therapy were not sufficient to prevent the development of chronic infection. It is suggested that complement may play an important role in the pathogenesis of C fetus infections.