An infant born prematurely (25wk;720g) presented at six months with acrodermatitis enteropathica. Despite adequate dietary zinc (zn), good weight gain and a normal stooling pattern, the serum zn level was 15 ug/dl (nl range 50-150 ug/dl). Lymphocyte response to the T cell mitogen PHA was normal, but the patient was anergic, and had abnormally high numbers of circulating B cells (1,847 cells/ul;nl 115/cells/ul). The serum IgG level was 140 mg/dl and IgA level was 6 mg/dl. No significant antibody to Tetanus, and no detectable antibody to Pertussis developed after primary immunization. Zn sulphate (2mg/kg/day) rapidly reversed the clinical condition, but immunological abnormalities responded slowly. After three months of zn replacement therapy (with adequate serum zn levels), the infant remained hypogammaglobulinemic, B cells in circulation remained elevated, and the response to the B cell mitogen Pokewood (PWM) was depressed. B cells, however, did respond in vitro to the T cell independent B cell mitogen Staph aureus Cowan (SAC). Responses to allogenic lymphocytes have shown consistent improvement. Overall, the absence of antigen specific responses, the decreased expression of the T3 marker, with normal proliferative responses to PHA and SAC, but not PWM, suggests that prolonged zn deficiency produces chronic disturbances in antigen presentation and/or T cell interactions that respond only slowly to zn replacement therapy. The selectivity of these immunological defects points to those areas of the immune system that may be critically zn dependent.