Ribotype patterns for L. monocytogenes include EcoR1 fragments with approximate sizes of 2000 bp, 2250 bp, and 3850 bp. These bands are chosen as naturally occurring internal standards for migration position normalization. A data set of 7 L. monocytogenes isolates with 9 replicates (3 patterns x 3 gels) for each isolate generates position-normalized densitogram peak report patterns per each isolate set. The 9 patterns for an isolate can be compared to any other isolate's set of 9 patterns to calculate "all-combinations" pattern matches between the sets of patterns. Between any two isolates' replicate pattern sets exists a quality of match distribution represented by a summary distribution location statistic, such as the median or mode. For a set of 7 isolates, the summary match quality statistics can be placed in a 7 x 7 similarity matrix and graphically represented as a dendrogram of isolate numerical relatedness. Qualities of match between the replicate pattern sets of isolates of the same strain generally yield values in the range of 0.90 - 1.00 depending on the summary distribution statistic chosen.