1. A poor hatchability syndrome, characterised by a high incidence of weak chickens, with clubbed down and long ginger hairs, in commercial broiler breeding stock was investigated.
2. If the weak chick were given a single oral treatment of ammonium molybdate (40 μg Mo/chick at 1 d), there was a reduction in subsequent mortality in both sexes and an increase in the growth rate of feathers in males, but not in females.