Techniques are presented for the statistical treatment of data obtained from single titration plaque count viral assays. It is recommended that the standard statistical analysis of any such assay include a test for Poisson distribution of the plaque counts and interval estimates for titer (in total and by virus type), and proportions represented by virus types in the sample. Techniques for improving the quality of the data are discussed, including suggestions to type all viruses from selected bottles rather than randomly selecting plaques for typing, sample size guidelines, and measures to be taken to optimize the precision of the assay.