Changes in localization of virus-associated antigens induced by F3TdR were studied using HSV-1 infected monolayer of cultured SIRC cells. A direct enzyme-antibody method was employed for preembedding for immunoelectron and light microscopy. At 3 hours post infection (h.p.i.), immunoreaction products began to appear in cytoplasm and nuclei in both the control and the F3TdR-treated group. The amounts increased during the subsequent hours of observation. In the control group, there was an intense diffuse immunoreaction in the cytoplasm and nuclei and dense accumulations of the reaction products were seen on the cell surfaces. In F3TdR-treated cells, intranuclear antigens were present as diffuse accumulations of granular deposits, and cytoplasmic stainings were very much reduced in degree; deposits on the cell surface were a rare finding. Progeny virus particles were significantly fewer in the presence of F3TdR; nucleocapsids were found but in a greatly reduced number and no enveloped particle was found. The morphology of individual nucleocapsid, however, was not significantly altered by treatment with F3TdR. A preponderance of antigenic substances in the nuclei over the cytoplasm in treatment group may reflect distorted protein synthesis and may provide morphological evidence of the biochemically assumed hypothesis of F3TdR efficacy; F3TdR exerts it's antiviral effects through abnormal protein synthesis and resultant defects in capsid assembly. Decreased antigenic substances in the membranous structures of cytoplasm may be related to reduced or abnormal production of viral antigens closely associated with envelopment.