To determine the influence of the antibiotics ceftiofur sodium from two different laboratories (A and B) and gentamycin sulfate on a Marek's disease commercial vaccine herpesvirus of turkey (HVT), samples were assayed by titration in chicken embryo fibroblasts (CEF). Viruses were tested in vitro to establish the average number of plaque-forming units before and after different periods of incubation with the addition of the antibiotic. These tests showed no effect of gentamycin or ceftiofur A or B on HVT titers when treatments were for 1 hr or less. However, ceftiofur B decreased the titer at 2 hr. The in vivo effects of the antibiotics were determined by vaccinating 15 one-day-old chickens with HVT plus gentamycin or ceftiofur A or B. Birds were considered viremic at 1 wk postvaccination when one or more plaques were detected in CEF 5 days after inoculation of peripheral blood lymphocytes. Viremia levels were similar between 1 and 16 wk after vaccination with HVT with ceftiofur A or B. The pH values (7.5) were the same in vaccines with and without antibiotics.