Extracts were prepared from the whole limb buds of the 11 day-old mouse embryos and from the postaxial or preaxial parts of the 12 day-old embryo limb buds. Effects of these extracts on the growth of the 13 day-old bpH/bpH embryo tibia rudiments were studied in vitro. It was shown that the limb buds of the 11- and 12 day-old mouse embryos contain a factor regulating bpH gene expression in differentiating cartilage cells. This factor is present in the postaxial part of the limb bud and is absent in its preaxial one. When the extract from the limb bud postaxial parts of the 12 day-old +/+ embryos is added to culture medium, the expression of bpH gene is observed in cultured tibia rudiments of the 13 day-old bpH/bpH embryos. The extraxt from the limb bud preaxial parts of the 12 day-old +/+ embryos does not affect the growth of tibia rudiments of mutant embryos in vitro. The regulator factor does not induce bpH gene expression in cultured bone rudiments of the 14 day-old bpH/bpH embryos. It is obvious that bpH locus does not act in cartilage cells of developing tibia after the 14th day of embryogenesis. The sensitivity of the factor regulating bpH gene expression to heating and proteolytic enzymes suggests its protein nature. The molecular weight of this protein is from 15 000 to 25 000. Experiments with actinomycin D show that this factor regulates the expression of bpH at the post-transcriptional level.