The effects of vitamin A excess of insufficiency on resistance to Escherichia coli infection and subsequent anti-E. coli immune response were examined in chicks. Chicks receiving depleted (0 microgram/kg), sufficient (0.85 mg/kg) or excess (1000 mg/kg) levels of vitamin A in their feed were inoculated by a subcutaneous injection of pathogenic E. coli (1 x 10(9) and 2 x 10(9) cfu per chick). Susceptibility to E. coli was determined by mortality, morbidity and immune responses (antibody production and T lymphocyte proliferation). Excess of insufficient vitamin A led to increased susceptibility of chicks of E. coli infection; this was accompanied by depressed immune responses. Chicks receiving excess vitamin A were more sensitive to E. coli than vitamin A-depleted chicks. This was reflected in higher mortality and morbidity rates and in severely depressed immune responses. In contrast to chicks receiving excess vitamin A, T lymphocyte responses (though not antibody responses) of vitamin A-depleted chicks achieved levels similar to those of vitamin A-sufficient birds with a lag period of 6 to 10 d. Therefore, reduction in resistance to E. coli infection, resulting from vitamin A excess or deficiency, probably was compounded by a delayed immune response.