Monoclonal antibodies (MAbs) to mouse interferons (MuIFN) have been used to characterize the interferon-like activities spontaneously expressed in mouse peritoneal macrophages freshly explanted from normal pathogen-free mice. Injection of mice with MAbs to MuIFN-alpha or -beta resulted in a significant increase of vesicular stomatitis virus (VSV) multiplication in peritoneal macrophages. Addition of these MAbs to freshly explanted mouse macrophages accelerated the decay of the antiviral state to VSV during the 'ageing' in vitro of these macrophage cultures. Furthermore, these MAbs to MuIFN-alpha or -beta markedly inhibited the transfer of the antiviral state from freshly explanted peritoneal cells or macrophages to syngeneic macrophages 'aged' in vitro permissive for virus replication. These effects were not observed using a non-neutralizing antibody to MuIFN-alpha, nor with a MAb to MuIFN-gamma. In all experiments sheep polyclonal antibodies to MuIFN-alpha/beta were more effective than the corresponding amount of MAbs to MuIFN-alpha or -beta. A mixture of both these MAbs was more effective than either alone. Interferons produced after stimulation of peritoneal macrophages with Newcastle disease virus (NDV) and of total peritoneal cells with lipopolysaccharides (LPS) have also been characterized by means of MAbs to IFNs. The results of neutralization studies with these antibodies indicated that MuIFN-beta was the major component of peritoneal cell IFN (induced by both NDV and LPS) and MuIFN-alpha was a minor component (13 to 17%). These data indicate that both MuIFN-alpha and -beta, but not MuIFN-gamma, are spontaneously present in/on mouse peritoneal macrophages and are produced after stimulation with NDV or LPS.