When polarizing mesoderm from the posterior border of the 4-day chick limb bud is placed adjacent to anterior limb mesoderm and ectodermal ridge, the anterior ridge thickens and mesodermal outgrowth ensues, resulting in supernumerary limb structures. This apposition of anterior and posterior limb tissues can be accomplished by cutting off the apical one third of the limb bud and reimplanting it on the stump with its anteroposterior axis reversed. The preaxial response to polarizing activity can be obtained after only 12--18 h in the reoriented position. Reversed apical mesoderm develops supernumerary digits when combined with untreated ectoderm. The reciprocal combination, reversed ectoderm and untreated mesoderm, fails to develop supernumerary structures. We have interpreted this as evidence that, in inducing supernumerary limb structures, polarizing activity acts only on the mesoderm.