The prevalence of antibodies to Brucella spp., Mycobacterium paratuberculosis and the Mycoplasma spp. causing contagious bovine pleuropneumonia and contagious caprine pleuropneumonia was determined in various species of ruminants on a ranch in the semi-arid zone of southeastern Kenya. Antibody titers to Brucella spp. were found in eland (Taurotragus oryx), oryx (Oryx beisa) and camels (Camelus dromedarius). Reactors were not found in buffalo (Syncerus caffer), sheep (Ovis aries) and goats (Capra hircus). Brucella sp. was not isolated from eland and camels. Antibody titers to M. paratuberculosis were found only in camels and goats. Mycobacteria were not detected in feces of two serologically positive camels. Significant serum antibody titers to Mycoplasma mycoides mycoides were found only in camels. Antibody titers to Mycoplasma sp. (strain F38), which causes contagious caprine pleuropneumonia, were found in buffalo, cattle and camels but not in the other species. Attempts to isolate the Mycoplasma sp. from nasal secretion of the buffalo and camels failed. The possible occurrence of tuberculosis in camels is discussed. Under the conditions at the ranch, contagious bacterial diseases appear to be of minor importance in the domesticated wild herbivores. The introduced camels, however, might be a source of various infections such as brucellosis, mycoplasmosis and possibly tuberculosis for the other susceptible species.