1. The effects of 5 and 25 mg/kg boron supplementation of diets with inadequate (6.25 μg/kg) or adequate (50 μg/kg) cholecalciferol (vitamin DS) content on some biochemical parameters, tibia characteristics, peripheral blood lymphocyte and splenic plasma cell counts of broilers were investigated. 2. Supplementation of the diet with boron affected plasma concentrations of boron, iron, copper and zinc and also tibia boron, zinc and calcium concentrations but did not have any effect on tibia iron or copper concentrations or tibia ash and tibia weight values. 3. Boron supplementation caused significant increases in splenic plasma cell count but decreased the proximal and distal tibia growth plate widths. There was no effect of boron supplementation on peripheral blood alpha-naphthyl acetate esterase (ANAE) content. Whole blood haematocrit and haemoglobin counts were significantly increased by boron supplementation but there were no effects on leucocyte ratios such as eosinophil, basophil, monocyte, lymphocyte and thrombocyte. 4. In general, the findings of the present study support the hypothesis that boron has an important biological role that affects the mineral metabolism of animals by influencing both biochemical and haematological mechanisms. © 2005 British Poultry Science Ltd.