The mechanism of iota-carrageenan (i-CAR)-induced protection against murine cytomegalovirus (MCMV) infection of mice was analysed. The virus titres in the target organs on the fourth day correlated with that in the plasma at 1 h after intraperitoneal inoculation of MCMV, irrespective of i-CAR treatment. Pretreatment of mice with i-CAR induced infiltration of polymorphonuclear neutrophils into the peritoneal cavity and inhibition of viral spread from the peritoneal cavity to the plasma. Although the direct relationship of these two phenomena was unclear, the inhibition by i-CAR of viral spread resulted in protection against MCMV infection of mice.