Natural killer (NK) cell activity was studied in iron-deficient rat pups. Pregnant dams were fed diets containing 6, 10 or 250 ppm Fe ad libitum from d 1 of gestation through d 21 of lactation. Two days post parturition litters were adjusted to seven pups each, and on d 17 the pups were injected intraperitoneally with 5 X 10(5) plaque-forming units of vaccinia virus. Following a 4-d incubation period, spleens were removed and the cell suspensions combined with YAC-1 target cells to measure cytolysis in a 4- and 16-h chromium release assay. Hematocrit levels of severe (6 ppm) and moderately (10 ppm) iron-deficient rat pups were significantly lower than that of controls. Similarly, body weight and spleen weights were significantly lower in iron-deficient pups than in control pups. Iron deficiency significantly impaired spleen NK cell activity when measured by two different effector:target ratios and assay time periods.