[E]-5-(2-bromovinyl)-2'-deoxyuridine (BVdU) inhibits the replication of the baculovirus Trichoplusia ni multiple nucleocapsid nuclear polyhedrosis virus in Spodoptera frugiperda cells. Virus-specific DNA synthesis and late protein synthesis are suppressed by the drug. BVdU is phosphorylated by deoxythymidine (deoxycytidine) kinase present in both uninfected and virus-infected cells, and in its 5'-triphosphate form it inhibits DNA polymerase activity in virus-infected cells. The effect of the BVdU is not completely reversible. Phosphonoacetic acid, phosphonoformic acid and Acyclovir have no effect on baculovirus replication. Acyclovir fails to compete with deoxycytidine and thymidine as substrates for pyrimidine deoxynucleoside kinase in virus-infected and uninfected cells.