Cultural and biochemical characteristics of Campylobacter fetus ssp. jejuni/Campylobacter coli strains of human and animal origin were studied and compared to those of reference C. fetus ssp. fetus, C. fetus ssp. intestinalis and C. fetus ssp. jejuni. Colony type and cellular morphology were determined by light and electron microscopy. Growth and biochemical responses to 21 separate tests were measured. Discrete and swarming colony types were encountered. Young culture cells were comma-shaped, S-shaped and spiral. In cultures over 72 h old, coccoid cells constituted the majority. With the available biochemical tests, strains of C. fetus ssp. jejuni/C. coli were distinctly different than other subspecies of C. fetus. They were also tentatively separated into 2 distinct groups and a third group with intermediate characteristics. Each group was further subdivided into subgroups. This study indicates that C. fetus ssp. jejuni and C. coli are very closely related and the tests employed in this study are not sufficient to separate them into 2 distinct species;Sixty-one test strains of C. fetus ssp. jejuni/C. coli of human and animal origin were subjected to serotyping by the passive hemagglutination technique. Rabbit antisera against live whole-cell antigens from Penner's reference serotype strains were titrated with sheep red blood cells sensitized with steamed antigens of the test strains. Forty-five of the test strains were found to be typeable by antisera against 17 of the Penner serotype strains. Among the typeable strains, certain serotypes (T8, T2, T1) were more common than others. This study shows that 79% of C. fetus ssp. jejuni/C. coli strains studied share common antigens with the Penner serotype strains of C. fetus ssp. jejuni;The pathogenicity of C. fetus ssp. jejuni and C. coli for the pregnant guinea pig was investigated in intraperitoneally inoculated pregnant guinea pigs. Abortion and cultural reisolation of Campylobacter organisms from uteri and visceral organs of inoculated animals were considered evidence for the pathogenicity of the strains being tested. Abortion rates ranging from 13%-86% were produced by the tested strains with a highest rate of reisolation from uteri of aborting guinea pigs. This study indicates that C. fetus ssp. jejuni and C. coli are abortifacient pathogens in the pregnant guinea pig.