This paper describes the construction of a recombinant Marek's disease virus serotype 1 (MDV1) in which the Escherichia coli lacZ gene was inserted into the open reading frame homologous to the US10 gene of herpes simplex virus 1 (HSV1). The recombinant virus replicated as well in cell culture as the parental MDV1 K-554 strain. Chickens immunized with the virus were protected against challenge with virulent MDV1, and produced a high level of antibodies against beta-galactosidase as well as against MDV1 antigens. The antibody titres persisted for at least 16 weeks. These results demonstrate that the US10 gene of MDV1 is an effective site for the insertion of foreign genes from which to construct a polyvalent live vaccine for poultry.