AbstractInvestigation into the rnechanism(s) whereby monocytes‐macrophages are mobilized to the myocardium of coxsackievirus (CVB3)‐infected mice was approached by assessing the role of anti‐idiotypic antibodies against specific antiviral IgG molecules. Balb/c mice preinocuiated with polyclonal anti‐idiotypic, syngeneic antibodies resulted in significantly decreased myocarditis after virus challenge. On the other hand, oil‐induced peritoneal exudate cells obtained from virus‐exposed animals demonstrated increased chemotaxis in the presence of polyclonal anti‐Id preparations, whereas peritoneal exudate cells from normal animals did not Specificity of the anti‐Ids as confirmed by the binding of the syngeneic anti‐anti‐idiotypic antibodies (antibody 3) to solubilized viral antigen(s) and Fab fragments prepared from the anti‐idiotypic antibodies. Paradoxic biological effects of anti‐Id, demonstrated in vitro and in vivo, suggest possible and subtle immunoregulatton of the macrophage effector arm by anti‐ldiotypes during the course of virus infection.