In a 6-week experiment on 560 Cobb broilers, the response of chickens to supplementation of diets with particulate grit and 25-hydroxycholecalciferol (25-OH-D₃) was evaluated. Maize- and soyabean meal-based diets contained pulverized limestone or a mixture of 70% pulverized and 30% particulate (1.2-4 mm in diameter) limestone as a source of calcium. Both diets were supplemented with vitamin D₃ (cholecalciferol) or 25-OH-D₃ was substituted in amounts equivalent to 20, 40, 50, 60, 80, or 100% of cholecalciferol. Incorporation of particulate forms of limestone into the diet positively affected performance and the ash and Ca contents in the tibia. Replacement of cholecalciferol by 25-OH-D₃ at a level of 50 and 60% had a beneficial effect on body weight gain and feed conversion, 100% substitution, on the Ca content of the tibia, whereas 50, 60, 80 and 100% substitution, on Ca and P balance.