Investigations of biocide efficacy by automated methods involving optical density measurements, e.g. using the recently published 'Bioscreen' method, are complicated by the fact that a poor correlation often exists between the log reductions obtained using the automated method vs those obtained by the traditional plate count methods. It was hypothesized that the differences observed between the two methods were due to the level of cell injury, which was masked by the optical density methods but which was recognized by the plate counts. Comparisons of log reductions following a disinfection test always showed the Bioscreen method to be overestimating the log reductions with respect to the plate counts. A correlation between colony size on the plates and the 'Bioscreen' results for a fixed disinfectant concentration and contact time was found using Global Imaging software. The results obtained also suggested that the observed colony area was dependent on the amount of injury incurred by a microbe during the disinfection process. A mathematical model of injury was developed which predicted the observed differences between the Bioscreen and the traditional plate method. The model further suggested a possible reason for biocidal lags.