The initiation of limb development depends on the site specific proliferation of the mesenchyme by the signals from the apical ectodermal ridge (AER) in embryonic mouse. We have previously reported that the local expression of Hst-1/Fgf-4 transcripts in AER of the mouse limb bud is developmentally regulated, expressed at 11 and 12 days post coitus (p.c.) embryo. In an effort to further understand the role of Hst-1/FGF-4 in mouse limb development, an antisense oligodeoxynucleotides (ODNs) study was performed. We first established a novel organ culture system to study mouse limb development in vitro. This system allows mouse limb bud at 9.5-10-d p.c. embryo, when placed on a sheet of extracellular matrix in a defined medium, to differentiate into a limb at 12.5-d p.c. embryo within 4.5 d. Using this organ culture system, we have shown that exposure of 9.5-10-d p.c. embryonal limb bud explants to antisense ODNs of Hst-1/FGF-4 blocks limb development. In contrast, sense and scrambled ODNs have no inhibitory effect on limb outgrowth, suggesting that Hst-1/FGF-4 may work as a potent inducing factor for mouse limb development.