Abstract Oster's turbidimetric technique for determining the pH of maximum aggregation of viruses was used in an attempt to differentiate four wild and nine mutant strains of tobacco mosaic virus. This involved the addition of dilute HCl to salt-free solutions of viruses until the maximum turbidity, as measured by a colorimeter, was reached. The pH values for the 13 strains used in the tests were within 1 pH unit range; those for Type and nine of its mutants were within a 0.22 pH unit range. Results were reproducible within 0.06 pH unit in tests of the same virus sample, at several concentrations, and for different preparations of the same strain. A mixture of viruses behaved like a single strain, giving the same type of turbidity curve with just as sharp a break; however, the pH of maximum turbidity of the mixture was just halfway between those for the separate strains. Thus, such a test does not distinguish a single strain from a mixture. Comparison of differences among strains in rates of movement in Turkish tobacco showed that the pH of maximum turbidity probably is not related to the time required for the virus to move from the inoculated mid-leaf to the top of the plant.