The infectivity titers of varicella-zoster virus (VZV) are routinely estimated by plaque production in cell culture. In this report, we show that plaque counts for VZV (strain Oka/Merck), in MRC-5 cell cultures, are significantly enhanced (54% average enhancement) by the use of an agarose overlay medium, as compared to a fluid overlay medium. Evidence also is presented that less variability (P less than 0.05) in plaque counts occurs with the use of an agarose overlay medium.