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Radclife Infirmary, Oxford THE method of Miles, Misra and Irwin (1938) has for many years been the standard technique for the determination of viable bacterial counts by surface inoculation of solid media. However, this method requires precision and skill if accurate results are to be obtained. One problem is the difficulty of preparing pipettes that accurately and consistently deliver 50 drops per ml.The method is also not without hazard to the operator. Johannson and Ferris (1946) have demonstrated aerosol formation when a liquid is dropped on to an agar surface, and the necessary agitation of dilutions during mixing also produces airborne bacteria.We describe and evaluate a procedure in which a semi-automatic pipette with glass capillary tubes is used. This method does not require the preparation and calibration of accurate dropping pipettes but has the accuracy of the Miles, Misra and Irwin technique; it also has the advantages of being faster and of producing less bacterial contamination of the working surface.MATERIALS AND METHODS Bacteria. Counts were performed on 24 bacterial broth cultures, each containing approximately lo6 organisms per ml. The cultures were all enterobacteria (table I). The organisms were identified by methods described by Cowan (1974). None of the strains tested was unduly mucoid.Culture media. Peptone water (Oxoid Bacteriological Peptone 1 %, with NaCl 05%) was used for the initial broth cultures and for subsequent dilutions. Counts were made on overnight broth cultures of the test organisms diluted approximately 1 in lo3. Blood agar plates (Oxoid Columbia Agar 2%, with defibrinated horse blood 5%) were used in the micropipette method and in the method of Miles, Misra and Irwin; plates were dried for 2 h at 37°C before use.In the pour-plate method, Columbia Agar (Oxoid) was used as the medium. As a less nutritious medium was used for the pour-plate method than for the other methods, a comparison of the two media was made by estimating the number of viable bacteria in 12 of the cultures listed in table I by the method of Miles, Misra and Irwin. The two media were inoculated in parallel. There was no significant difference between the plate counts obtained with the two media (t = 0.73, df = 11, P > 0.05).