Tandemly reiterated sequences represent a rich source of highly polymorphic markers for genetic linkage, mapping, and personal identification. Human trimeric and tetrameric short tandem repeats (STRs) were studied for informativeness, frequency, distribution, and suitability for DNA typing and genetic mapping. The STRs were highly polymorphic and inherited stably. A STR-based multiplex PCR for personal identification is described. It features fluorescent detection of amplified products on sequencing gels, specific allele identification, simultaneous detection of independent loci, and internal size standards. Variation in allele frequencies were explored for four U.S. populations. The three STR loci (chromosomes 4, 11, and X) used in the fluorescent multiplex PCR have a combined average individualization potential of 1/500 individuals. STR loci appear common, being found every 300-500 kb on the X chromosome. The combined frequency of polymorphic trimeric and tetrameric STRs could be as high as 1 locus/20 kb. The markers should be useful for genetic mapping, as they are sequence based, and can be multiplexed with the PCR. A method enabling rapid localization of STRs and determination of their flanking DNA sequences was developed, thus simplifying the identification of polymorphic STR loci. The ease by which STRs may be identified, as well as their genetic and physical mapping utility, give them the properties of useful sequence tagged sites (STSs) for the human genome initiative.