ABSTRACT Mouse primordial germ cells express tissue non-specific alkaline phosphatase (TNAP) during development, but the widespread expression of another alkaline phosphatase gene in the early embryo limits the potential use of this marker to trace germ cells. To attempt to identify germ cells at all stages during embryonic development and to understand the role of TNAP in germ cell ontogeny, mice carrying a βgeo (lacZ/neor) disrupted allele of the TNAP gene were generated by homologous recombination in embryonic stem cells. Using β-galactosidase activity, the embryonic pattern of TNAP expression was examined from the blastocyst stage to embryonic day 14. Results indicate that primordial germ cell progenitors do not express TNAP prior to gastrulation although at earlier times TNAP expression is found in an extraembryonic lineage destined to form the chorion. In homozygous mutants, primordial germ cells appear unaffected indicating that TNAP is not essential for their development or migration.