A study was conducted on the impact of aflatoxin in the feed on the prophylactic immunization of layer hens against Newcastle disease, infectious bronchitis, infectious bursal disease and fowl cholera. Four-hundred-and-eighty 18-week-old white leghorn chickens were used. Different groups of hens were vaccinated, as per commercial recommendations, with a commercial inactivated triple vaccine against Newcastle disease, infectious bronchitis, and infectious bursal disease. A killed polyvalent bacterin was used for fowl cholera. Aflatoxin was fed for 22 weeks at a daily dose of 200 parts/10(9)/hen. Aflatoxin significantly reduced antibody titres, resulted in a decrease of egg weight, a decrease in egg production and an increase of mortality rate in challenged hens. Aflatoxin was detected in eggs at levels far above the permissible concentration.