When normal mice are challenged intravenously with Corynebacterium equi, the organisms are generally cleared from the liver, spleen and lung after an initial increase in numbers in these tissues. Viable organisms can still be recovered from the liver and spleen 5 weeks after challenge. The animals show no sign of infection even when tissue counts exceed 1 X 10(7). In-vivo growth rates of C. equi are similar when the challenge inoculum is 6 h and 21 h old. In-vitro growth temperature does not affect the pattern of in-vivo growth and clearance. There was no evidence of systemic involvement when 6 X 10(7) and 7 X 10(7) organisms were administered orally and subcutaneously, respectively. Following intranasal challenge with 1.5 X 10(6) organisms mice can clear the lungs within 4 weeks, but when challenged with 10 times this number of organisms, the clearance rate is slowed markedly and pneumonia results. Administration of carrageenan before bacterial challenge reduces clearance rates from the tissues but does not affect mortality rates. Cyclophosphamide treatment with C. equi infection converts a non-lethal challenge into a fatal infection and impairs clearance from the tissues.