A viral passage method using sonication to obtain cell-free virus was compared with the conventional viral cell culture passage technic. The recovery of 121 varicella zoster virus and cytomegalovirus isolates in human fibroblast vials from sonicated versus nonsonicated passage suspensions was studied. These fibroblast vials with cytopathic effect were identified using monoclonal fluorescent antisera. Twenty-eight (29%) of cytomegalovirus isolates were recovered only from sonicated passages, and an additional 28 isolates had faster recovery and/or a greater number of viral plaques from sonicated passages. Sonication did not increase significantly the varicella zoster virus recovery rate.