The ability of eight neutralizing monoclonal antibodies (MAbs) specific for structural proteins of murine cytomegalovirus (MCMV) to protect mice passively against MCMV infection was examined to determine firstly whether a correlation existed between the neutralization titres of the MAbs in vitro and the protection afforded by the MAbs in vivo and, secondly, the contribution of the host towards neutralization by the MAbs in vivo. The reduction in MCMV titre in the livers of BALB/c and C57BL/10 mice by the MAbs closely correlated with their neutralization titres in vitro. However, in the spleens of BALB/c mice, in which MCMV replicates to high titre, almost all of the MAbs tested were ineffective in reducing MCMV replication. Indeed, a significant increase in splenic MCMV replication was observed in mice treated 24 h prior to MCMV replication with either neutralizing MAbs or polyclonal Ig. Each of six MAbs prophylactically protected between 66 and 100% of mice from an intraperitoneal challenge with 4 LD50 MCMV regardless of their neutralization titre in vitro. The persistence of MCMV replication in the salivary gland was not prevented by either polyclonal Ig or MAbs. Despite the absolute requirement for complement for the neutralization of MCMV in vitro, both polyclonal Ig and MAb 4F9 protected A/J mice, which are deficient in the fifth component of complement, as efficiently as they did complement competent BALB/c mice. These results demonstrate that MAbs specific for single MCMV polypeptides are protective in vivo. In addition, the extent to which the MAbs protected against MCMV could not be predicted from their immunoreactive or neutralizing titres in vitro or by their effect on splenic MCMV replication in vivo. Furthermore, these studies suggest that the mechanism(s) of neutralization of MCMV in vitro are different to those which act in vivo.