Twenty-four control cases of reactive follicular hyperplasia (RFH) and 28 cases of B-cell non-Hodgkin's malignant lymphoma (ML) were evaluated for surface immunoglobulin (sIg) light chain expression with flow cytometry. Because there was no significant difference between the distribution of the ML kappa/lambda ratios for which kappa/lambda greater than 1 compared with those ML lambda/kappa ratios for which lambda/kappa greater than 1 using the Kolmogorov-Smirnov statistic, the authors treated kappa/lambda and lambda/kappa data symmetrically. By transforming all kappa/lambda ratios to maximum ratios, that is, Rmax = max (kappa/lambda, lambda/kappa), so that all resulting ratios were at least one, the authors defined a new statistic for evaluating sIg light chain restriction. There was a highly significant difference for equality of distribution of the data using the Mann-Whitney test (P less than 0.0001). By choosing cut-points based on the Rmax statistic, the authors were able to separate the RFH and ML cases with sensitivity = 0.96 +/- 0.07 (95% confidence) and with specificity = 0.91 +/- 0.11 (95% confidence). These point estimates of sensitivity and specificity appear to be promising for the clinical utility of the Rmax statistic, even for cases of B-cell ML with discordantly low expression of sIg light chains. The Rmax statistic will facilitate the comparison of data between different laboratories because it does not require prior assignment of cut-points and can be made using a simple nonparametric test.