Estimation of streptomycin and penicillin in blood.

The method here described, which we have been using for some time in routine work, is a modification of the micro-method of May and others (1947), in which dilutions of the patient's serum are added to phenol-red-serum water on waxed slides, and incubated in capillary tubes. The dilutions of patient's serum and their addition to the medium are performed on a waxed glass plate which is incubated directly, without filling the mixtures into capillary tubes and fixing to plasticine-covered slides. In this way the amount of manipulation is reduced and an appreciable saving of time is achieved, especially when several sera are being titrated at the same time. Moreover the colour change at the end point is even more obvious than in capillaries and there is no danger of upsetting the tests by rough handling.