SUMMARY.The precautions which, should be observed in practical application of Holmes's local lesion method in quantitative work with tobacco mosaic have been examined. The aim has been to obtain results with the greatest statistical significance consistent with the use of reasonably small numbers of plants. The adoption of a standard method of inoculation, and the comparison of viruses on opposite halves of the same leaves, were found to be the most important points to observe, and a method of work has been outlined.Tomato spotted wilt forms necrotic primary lesions on the leaves of tobacco of a character suitable for quantitative work, and a number of the results obtained with tobacco mosaic have been checked with this virus.Applications of the method are discussed. It is shown that rapid comparison of virus concentrations from different sources should be possible after further work has been done on dilution curves. An illustration is also given of the application of the method in plotting the curve of “ageing,” or change in virulence with time, of the expressed virus of tomato spotted wilt. This curve was found to be of the logarithmic t