Short tandem repeat (STR) loci are routinely employed for individual identification. WE have examined the performance and reproducibility of a highly informative co-amplification system containing the tetranucleotide STR loci: HUMVWFA31/A, HUMTH01, D20S85, D8S1179, HUMFIBRA, D21S11, and D18S51, in conjunction with the amelogenin sex test, in addition to a modified system omitting the locus D20S85. Polymerase chain reaction (PCR) products were fluorescently detected on an automated sequencer and automatically sized against an internal size standard by Genescan software. Both systems were routinely able to type 500 pg of undegraded DNA. At DNA concentrations between 50-500 pg, partial profiles were produced, but no allelic drop-out was observed. Balanced amplification of all loci occurred over a wide range of DNA concentrations from 50 pg to 10 ng. Alteration of reagent concentrations and cycling parameters from optimal resulted in variation in the efficiency of individual locus amplification relative to the other loci within the system. This was also observed at high ionic strength or extreme pH. However, at all reagent concentrations and conditions, allelic drop-out was not observed. These multiplex systems have potential in both routine forensic and intelligence database applications.