An experiment was conducted to measure the influence of a dietary probiotic on the production performance, the concentrations of cholesterol in the serum and yolk and immune competence in White Leghorn layers from 25 to 72 weeks of age. One hundred and twenty commercial White Leghorn layers, aged 24 weeks, with an average of 62% hen-day egg production, were equally and randomly distributed into three groups, with eight replicates of 5 birds in each. The birds were reared in individual laying cages. They were placed on one of three dietary treatments: basal, or basal with probiotic supplementation at a rate of 100 or 200 mg/kg feed. The addition of probiotic significantly increased the egg production, shell weight, shell thickness and serum calcium, and reduced the concentrations of cholesterol in the serum and yolk. However, no differences in these traits was observed between the groups receiving 100 or 200 mg probiotic. Feed conversion, egg weight, serum phosphorus and serum alkaline phosphatase activity were not influenced by supplementation with probiotic. Antibody production in response to the inoculation of sheep red blood cells and the cutaneous basophilic hypersensitivity (CBH) responses to inoculation with phytohaemagglutinin did not differ significantly among the dietary groups at either 24 or 40 weeks of age. The antibody titre was significantly higher in the groups supplemented with probiotic at 64 weeks of age. The addition of 100 mg/kg of probiotic in the diet significantly increased the CBH response at 64 weeks of age.