This study was undertaken to assess the effects of dietary ascorbic acid on the growth and immunoresponsiveness of chickens when subjected to particular types of stress. White Leghorn chicks were fed diets containing no supplemental ascorbic acid, and ascorbic-acid diet (330 ppm) for 2 days or for 19 days before challenge. Then, half of the females were inoculated with Escherichia coli; half of the males were challenged with dietary corticosterone (30 ppm) for 12 days; and the remaining chicks were maintained as controls. These chicks, reared under "good" husbandry procedures, did not realize advantages in growth or feed efficiency due to the short- or long-term consumption of diets containing ascorbic acid. Incubation with E. coli resulted in considerably higher heterophil-to-lymphocyte ratios 24 h after inoculation, and E. coli-induced mortality was higher for pullets on short-term ascorbic acid than for those on long-term or no ascorbic acid. Dietary corticosterone caused differences in body weight and the relative weights of certain organs, regardless of dietary levels of ascorbic acid. The antibody response to red-blood-cell antigens from sheep was enhanced in unchallenged cockerels (no dietary corticosterone) fed ascorbic acid on a long-term basis, but dietary corticosterone overshadowed the advantageous effects of dietary ascorbic acid. These data showed that the effects of supplemental ascorbic acid on growth and on immunoresponsiveness were related to the quality of the husbandry, length of supplemental feeding, age of the chicks, endogenous-exogenous balance for ascorbic acid, and the relationship with corticosterone.