Cell-free and cell-associated Marek's disease vaccines prepared from the TK/A isolate of the herpesvirus of turkeys (HVT) were compared to evaluate their relative effectiveness in protecting chicks with homologous maternal antibody. The influence of early challenge on protection was also investigated. Although the higher susceptibility of cell-free HVT to neutralising antibody could be demonstrated in vitro, no significant difference between the two types of vaccine could be established in vivo using chicks with maternal antibody. Chicks exposed to challenge immediately or only a few hours after vaccination were not adequately protected. A higher level of protection was observed when challenge was separated from vaccination by an interval of several days. The importance of proper vaccination procedure, vaccine virus titre and adequate management of vaccinated stock, with particular reference to the risks of early challenge are discussed.