Mesenchymal cells were isolated from the morphogenetic field of the wing bud of the Valo chicken at embryonic stage 20 and seeded into primary clonal culture. Primary clonal populations are serially subcloned for up to five times until heterogeneous or homogeneous clonal populations made up of cells with the characteristic morphology of fibroblasts, chondroblasts, and myoblasts could be distinguished. Primary and secondary clones studied showed the same (35S) methionine polypeptide pattern of the nuclear and cytoplasmic fraction, indicating that they are made up of identical cells, the stem cell S1. Tertiary clones exhibited three polypeptide pattern, one of the stem cell S1 and two new ones characterizing the stem cells S2a and S2b. In the fourth and fifth subcloning steps, clones with up to six different polypeptide