Although the kits apparently performed similarly with respect to specificity for male DNA when they were used in the 16 laboratories participating in the project, evaluators observed a significant variation in peak heights among the laboratories. This was observed for kits distributed by companies whose products are widely used (AB, Promega), as well as for products of companies that serve only the Middle-European market (Biotype, SERAC). The evaluators also observed that the overall peak heights were low. This means that if a minimal threshold value were to be applied to allele calling, as is the case for autosomal short tandem repeat (STR) typing, a large number of alleles would have been excluded from the report. Variation in the quantity and quality of the DNA shipped to the laboratories can be excluded as the reason for the observed difference, because samples and shipping conditions were determined to be reproducible in an earlier study. The DNA mixture samples were prepared by the organizing laboratory and thoroughly tested prior to shipping for the experiments. These findings show the need for an individual adaptation/optimization process in order to achieve comparable and optimal kit performance across laboratories. The following Y-STR typing kits were used in the main study: Y-filer (Applied Biosystems, AB, Foster City, CA); Argus Y Nonaplex (Biotype, Dresden, Germany); Powerplex Y (Promega, Madison, WI); and DYSplex-3 (without Amelogenin, SERAC Bad Homburg, Germany). 2 tables, 1 figures, 5 references, and appended list of participating laboratories and collaborating persons