Resistance to infections inducing two types of immune response, humoral and cell-mediated, has been measured in mice after Salmonella typhimurium and Klebsiella pneumoniae inoculation; the animals exhibited different kinds of obesity: genetic, ob/ob and db/db mutants, induced by fat diet or gold thioglucose (aurothioglucose) injection (determining obesity of central origin). Klebsiella infection was aggravated in all types of obesity. Salmonella infection was aggravated in genetically diabetic and dietary-obese mice. The two kinds of genetically obese mice show an important functional decrease in splenic lymphocytes. In contrast, aurothioglucose-obese mice were more resistant than controls.