The safety of cefoxitin, in terms of values obtained in laboratory tests during and after therapy, was estimated by three methods for analysis of data derived from controlled clinical comparisons of cephalothin and cefoxitin. Both antibiotics were found to be safe with respect to hematologic, renal, and hepatic function and did not differ significantly from each other. Laboratory data confirmed by tests performed serially and by paired related tests were analyzed by a novel method of comparison.