The development of efficient methods for establishing germline competent chicken embryonic stem (ES) cell lines has proved elusive. In the mouse embryo, expression of oct 3/4 is limited to pluripotent cells and primordial germ cells; regulatory sequences of this gene have been used to derive germline competent mouse ES cell lines by the continuous ablation of differentiated cells in culture using drug selection. To apply this technique to chickens several strategies were employed to analyze the chicken genome for oct 3/4, a member of the highly conserved POU gene family. PCR and Southern hybridization experiments with primers and probes based on mouse oct 3/4 sequences indicated that oct 3/4-like sequences are not present in the chicken genome. Also, analysis of mRNA from Stage 14 and 20 (H&H) chick embryos by reverse transcription PCR and the screening of a Stage 20 (H&H) chick embryo cDNA library with mouse oct 3/4-based primers and probes indicated that oct 3/4-like sequences are not expressed in the early chick embryo. The apparent absence of oct 3/4 in chickens, despite the conservation of the gene in mammals and urodeles, is discussed in terms of possible implications for the mode of chicken PGC formation in relation to that in other vertebrates.