The number of plaque-forming cells and its "normal range".

The number of antibody-forming cells is commonly assessed by the plaque technique. In this paper, two questions are discussed, connected with presenting the number of plaque-forming cells found in experiments: (i) the form of probability distribution of the number of plaque-forming cells (PFC), and (ii) determination and use of the so-called "normal range" of the number of PFC. Gottlieb's finding (1974) that the lognormal distribution is a good approximation to the distribution of the number of PFC is supported, and it is suggested that the upper one-sided tolerance limit be used as the limit of normal range of the number of PFC. A numerical study based on 1,450,000 pseudo-random experiments shows that it is important to make a correct assumption about the form of the distribution before a statistical method is used.