Dissociated cells of whole embryos or their animal halves were cultured until the time when sibling embryos reached the neurula stage, and expression of genes for embryo-specific epidermal XK81 keratin, muscle-specific α-actin and histone H4 was studied by Northern blot analysis. In the experiments with dissociated midblastula cells, the expression of keratin gene, but not two other genes, was found to be inhibited in animal cap cells when these cells were cultured with vegetal cells. However, in the experiments with midgastrula cells, such inhibition was not observed even when animal cap cells were co-cultured with vegetal cells. Therefore, keratin gene expression in animal cap cells is sensitive at the midblastula stage, but resistant at the midgastrula stage, to the inhibitory activity by vegetal cells. To determine when such transition occurs, we prepared animal cap cells from embryos at different stages and co-cultured them with vegetal cells from midblastulae. Results obtained showed that the transition takes place at the late blastula stage (stage 9). These results suggest that the transcriptional system for the embryo-specific epidermal XK81 keratin gene is stabilized in animal cap cells prior to gastrulation, so that it is expressed autonomously, without being inhibited by contact with vegetal cells.