The role of the apical ectodermal ridge and of fibroblast growth factors FGF-2 and FGF-4 and of the insulin-like growth factor I (IGF-I) in the control of the migration of epidermal melanoblasts was investigated using quail-chicken chimeras. Wing buds of a strain of unpigmented chicken were microsurgically modified in several ways (ablation, displacement or implantation of additional apical ectodermal ridges, implantation of grafts devoid of apical ectodermal ridges, ectopic application of growth factors) and received grafts containing quail neural crest cells. The distribution of the epidermal melanoblasts which had differentiated from the quail grafts revealed that both the apical ectodermal ridge and the growth factors invariably caused the migration of epidermal melanoblasts towards them. This leads to the conclusion that the presence of the apical ectodermal ridge is the sufficient condition to direct the migration of epidermal melanoblasts within the avian embryonic wing bud. Furthermore, FGF-2 and IGF-I and to a lesser extent FGF-4 play a decisive role in directing the migration of epidermal melanoblasts within chicken wing buds and are likely to be involved in the molecular cascade by means of which the apical ectodermal ridge controls the migration of epidermal melanoblasts.