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CSF gentamicin level of 1-2 mg/l (range 0 2-2 9) after intramuscular administration of 1-5-2-5 mg/kg. The same authors measured 2-6 mg gentamicin/l (range 2.5-2.7) in CSF 24 hours after an intrathecal dose of 0 5 mg, and 15-8 mg/l (range 4 3-60) after an intraventricular dose of 0 5-1 mg. CSF levels of gentamicin up to 7 mg/l were obtained by Newman and Holt (1967) after 1-2 mg/kg per day intramuscularly and 0 2-1 mg/day intraventricularly.It is well documented that the action of gentamicin is particularly susceptible to changes in the salt concentration of the medium (Rubenis et al, 1963;Schoutens and Yourassowsky, 1972). When it is necessary to assay the drug in body fluids this is usually done against reference standards which are made up in the same fluid in order to minimize any discrepancies due to variation in the constituents of the medium. However, this principle appears to have been neglected in assays of gentamicin in CSF, probably due, at least in part, to the fact that pooled human CSF is not readily available in many laboratories.Received for publication 20 January 1976 Reported levels of gentamicin in the CSF are often the result of assays against reference standards made up in water (Newman and Holt, 1967). In other cases the assay method is not described in sufficient detail, and this applies particularly to assays performed by a reference laboratory.

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(1971). The advantage of these methods is that results can be obtained within about two hours, but they require specialized equipment.Therefore most laboratories still use conventional plate diffusion methods.Since any microbiological method is subject to a considerable amount of error it is important to perform the assay under optimum conditions. In this article the possiblity of increasing the accuracy of the plate diffusion method by altering the constituents of the assay medium has been investigated. The Bacto antibiotic medium 2 (Difco) was used as the assay medium, having a final pH of 6-6. This was supplemented with 0-5%, 1%, 1.5%, and 2% sodium chloride where appropriate. The pH of the medium was adjusted by the addition of concentrated sodium hydroxide immediately before pouring the plates in order to prevent hydrolysis during autoclaving. PLATE DIFFUSION ASSAYTwenty-five millilitre volumes of assay medium were dispensed into 9 cm diameter plastic Petri dishes, giving a depth of about 4 mm. A sample of the overnight culture was diluted in distilled water and plates were surface seeded with this suspension. GENTAMICIN STANDARDSGentamicin sulphate (Genticin Injectable, Nicholas Laboratories) standards were prepared at concentrations of 2-5, 5, 10, 20, and 40 ,ug of gentamicin base per millilitre in pooled human serum and stored frozen. Results EFFECT OF SODIUM CHLORIDE ON THE STANDARD CURVEEach plate was seeded with approximately 2 x 106 organisms. Six wells, 9 mm in diameter, were cut in each plate. To each of five wells was added 0-15 ml of standard, one well remaining empty. Three replicate plates were used for each salt concentration.