Over several months, a dairy farm experienced high neonatal mortality in Holstein calves. Calves under 2 weeks of age developed signs including inappetance, diarrhea, neurologic disturbance, temperatures between 105 degrees F. and 100 degrees F, seizures, and death. In early affected calves, E. coli susceptible to enrofloxacin was cultured from liver and lungs; subsequently all calves born were treated with enrofloxacin and penicillin. This intervention was unsuccessful. on-farm investigation found calf management adequate. Drinking water was available after three days of age. Colostrum and environmental samples were submitted for culture; no pathogens were grown. A 4-day old calf presented to Cornell obtunded with a temperature of 106 degrees F. Serum sodium at admission was 180 mEq/L. Toxicological analysis of brain tissue from two necropsied calves found elevated sodium. Analysis of milk replacer found sodium 130-144 mEq/L and osmolality 800-900 mOsm/kg H2O. Industry standards recommend extreme caution when feeding sodium in excess of 120 mEq/L or osmolality in excess of 600 mOsm/kg H2O. The milk replacer brand was changed, neonatal mortality decreased, and the situation is considered resolved.