A new cephalosporin, cefonicid (1 g), was given intramuscularly to 49 patients 1 hr before they were to undergo surgery and to 10 healthy lactating women. The concentration of cefonicid was assayed by disk agar diffusion with the use of Bacillus subtilis as the test organism. Concentrations of cefonicid in tissue and fluid specimens were obtained. The data demonstrate that within 1 hr of intramuscular injection of cefonicid, effective concentrations of cefonicid in serum and tissue for common microbial pathogens were achieved. This finding suggests that cefonicid would be useful for perioperative prophylaxis in surgical patients. Although the concentration of cefonicid in breast milk was low at 1 hr after injection, more information is needed regarding the subsequent secretion of cefonicid before a conclusive statement can be made concerning the danger of sensitization in infants of nursing mothers.