The degree of histological deterioration of the original explant and the extent of cell spreading was evaluated in cultures of pectoralis muscle from 11-day chicks. Although the frequencies of these two parameters varied with the amounts of horse serum and embryo extract added to the medium, cultures from dystrophic chicks, in comparison to those from either normal or heterozygous animals, consistently showed the largest number of explants with the most extreme forms of histological deterioration and cell spreading. At 20 per cent horse serum the cultures from heterozygous chicks showed greater frequencies of the more extensive forms of deterioration and spreading than the normal muscle explants, but at 5 per cent horse serum these two groups appeared similar. Regardless of genetic background, cultures of the pectoralis muscle from 18-day embryos and of the latissimus dorsi muscle from 11-day chicks exhibited comparable high frequencies for the maximal degrees of deterioration and spreading.