SUMMARY The methods incorporated in the computer program used in a trial of computer- aided identification of bacteria are described. The identification method is based on Bayes's theorem and allows for dependent tests and missing data in the probability matrix. It was found useful in developing the method to take account of the occur- rence of errors in bacteriological testing. The method suggests a definite identifi- cation only if the Bayesian probability of one of the taxa exceeds a threshold level; if not, a separate procedure selects the best tests to continue the identification.