Intravenous infection of Swiss mice with a strain of Yersinia enterocolitica unable to colonize normal mice by the oral route, induced a systemic infection. Viable bacteria were isolated from homogenates of liver, spleen and lungs, as early as one hour after the challenge and have been detected during two weeks. On the other hand, the number of viable bacteria isolated from the blood has always been very low. Faecal samples permitted the isolation of Y. enterocolitica even after two weeks, while at this time no more bacteria were isolated from the tissues. The duration of the infection remained unchanged with an inoculum of 10(3), 10(4) or 10(5) viable bacteria. Depending on the size of the inoculum, liver macroscopic abscesses occurred more or less rapidly after the challenge. These abscesses disappeared spontaneously. A single intravenous injection of cyclophosphamide (200 mg/kg) six days after the challenge was followed by an important rise of the number of bacteria in all the tissues. Comparative studies of intravenously infected athymic (Nude) mice and controls showed a higher number of bacteria in the liver and spleen of the athymic animals.