An interlaboratory trial was made, by three analysts in each of the 19 laboratories, of three International Standards Organisation (ISO) colony count methods for aerobic organisms (ACC), Enterobacteriaceae (ECC) and Escherichia coli (EcCC). All tests were done in duplicate and were further replicated by plating both on culture media supplied by the organisers and on each laboratory's own choice of culture media. In order to avoid any influence of food matrix on the results, the inoculum for each test was a freeze-dried ampoule of a standardised mixed culture. After collation of test results, individual data sets were examined for obvious non-consistency, and colony counts for each individual test were determined both as simple and 'weighted' mean values. The derived colony counts were then log(10)-transformed and examined statistically. Estimates of repeatability and reproducibility for each set of results were derived and used to calculate the parameters for the uncertainty of measurement. Estimated values of the uncertainty of reproducibility and repeatability for the ACC ranged, respectively, from 9.3 to 12.1% and 2.0 to 3.9% of the mean log(10) colony count, depending on the specific culture media, the method of deriving the mean count and the statistical procedure employed. Similarly, estimates of the uncertainty of reproducibility and repeatability for the ECC ranged, respectively, from 14.0 to 17.4% and 4.1 to 6.7%. The estimates of uncertainty of reproducibility and repeatability for the EcCC data ranged, respectively, from 21.1 to 30.9% and 6.7 to 15.4%. Whilst the uncertainty estimates for the ACC data conform to long-held views on the repeatability and reproducibility of microbial count data, the estimates for the ECC and EcCC are considerably greater. It was notable that no benefits were seen in the use of the weighted mean as compared to simple mean colony count. The importance of uncertainty estimates of colony count data in the assessment of the microbiological quality of foods is discussed.