Reference ranges for two well-recognized tests for the lupus anticoagulant were determined utilizing 98 healthy subjects. The purpose of the study was to compare the reference ranges for the dilute tissue thromboplastin inhibition test on this group of healthy subjects calculated by parametric and nonparametric statistical methods, and to compare these results with results obtained on subsets of 20 and 40 randomly selected individuals from the group of 98. The same procedures were followed for the dilute Russell's viper venom test. Results were recorded in seconds of clotting times and in ratios (subject/mean of that set or subset). Statistical analysis revealed Gaussian distribution of the results in the large group as well as in each subset for both tests. The results showed more variation between sets of the dilute tissue thromboplastin inhibition test than of the dilute Russell's viper venom test. Nonparametrically calculated reference ranges were wider than those determined by a parametric method, especially if confidence intervals are provided for both reference ranges in the group of 94 controls or in a subset of 40 subjects. The nonparametric technique utilizes all data for the calculation of reference ranges of such sample sizes no matter how wide the results are spread. There was no significant difference between the reference ranges of subsets and the whole group (p > 0.05) calculated by both statistical techniques.