Spinal cord chimeras were produced by replacing a small fragment of neural tube of a 2-day-old White Leghorn chicken embryo with a similar fragment from a Japanese quail embryo. The embryo mortality was 61%, and 72% of hatched birds were 'cripples' and had to be sacrificed within 5 days after hatching. Forty-nine chimeras, 10.9% of the total number of operated embryos, were alive for more than 3 weeks. For at least 17 days after hatching, all birds behaved like normal chicks, and the grey quail-like feathers were the only manifestations of their chimerism. Initial neurological symptoms of unsteady walking and drooping of the wings were noted in all birds except for 1 that died an accidental death before it became sick. Advanced symptoms characterized by paralysis of the legs forcing the bird to lie on its side were noted in 40 birds. The chimeras could be divided into two groups, each consisting of 24 birds. The short-survival (SS) chimeras of the first group became terminally ill and had to be sacrificed within 3 months. The long-survival (LS) chimeras of the second group showed more protracted disease, in that only 16 of them showed symptoms of the advanced disease, and the majority showed partial or complete recovery. Ten of the LS birds were kept alive for more than 8 months. Furthermore, many LS chimeras lost their grey feathers. The hallmarks of neurohistological manifestations were mononuclear cell infiltrates, demyelinization with preservation of axons and scar formation. These lesions were restricted to the quail fragment of the spinal cord except for 2 birds in which distant cellular infiltrates were observed. Direct immunofluorescence tests for chicken IgG were positive in spinal cords of most SS chimeras but only of some LS chimeras.