Methods for the rapid determination of antibiotic blood levels require particular organisms or special conditions or long incubation. A method was developed for the rapid determination of antibiotic blood levels which requires no special equipment and is rapid. Special stains are not required. Utilizing bacteria from standard Kirby-Bauer sensitivity plates, a suspension is prepared and a pour plate made. Organisms which are sensitive to the questioned antibiotic and resistant to other antibiotics the patient is receiving are chosen. Muller-Hinton is the best general agar. Wells are cut, and antibiotic standards, antibiotic controls, and the patient's serum introduced. Within three to four hours, zone sizes can be read and the antibiotic blood level calculated. The materials and technical facility required should not preclude any laboratory from successfully performing the method.