In the past, it has been tacitly assumed that the distribution of the plaque-forming cell response in mice is approximately normal; that is, it would provide the usual bell-shaped normal distribution curve if enough replicate samples were collected. We address this point directly, by using the plaque-forming cell response generated in a total of five in situ, in vivo, and in vitro experimental systems. The Gaussian distribution assumption was tested on the untransformed data and on the same observations transformed by logarithms or square roots. We conclude that the log transformation provides a close approximation to normality for all the systems studied.