Of 655 bacterial strains isolated in a routine clinical diagnostic laboratory, 564 (86 per cent) were found sensitive to cephalothin by disk sensitivity test. However, the fraction of sensitive Gram-negative bacilli and enterococci declined sharply at concentrations approximating those obtained in serum after the usual recommended dosages. Gram-positive bacteria remained sensitive throughout the entire range of testing concentrations. Unless very high dosages are used, cephalothin cannot be considered a broad-spectrum antibacterial agent for use while cultures are pending. Even with very high dosages all strains of Pseudomonas spp. and significant fractions of other Gram-negative bacillary species remain resistant to cephalothin. If used alone, the high potency cephalothin disk may yield inadequate information and should be supplemented by a lower potency disk, standardized measurements of zone diameters, or a screening plate.