CTG trinucleotide repeat length polymorphism within the 3' region of the myotonin protein kinase I (MP-I) gene was examined with the use of the polymerase chain reaction (PCR) technique. A total of 159 DNA samples from healthy donors from five ethnic groups including Russians (n = 33), Azerbaijanians (n = 29), Uzbeks (n = 31), Moldavians (n = 31), and Georgians (n = 35) were analyzed. The number of CTG repeats varied from 5 to 24, and allele distribution was bimodal in all populations; alleles with 6, 7 and 9 repeats were not found. Statistical treatment of the data was performed by the contingency table collapsing procedure. Allele distribution in the populations of Russians, Azerbaijanians, Uzbeks and Georgians appeared to be statistically homogenous and significantly (P < 0.001) different from that in Moldavians as well as in Japanese, African, and Western European populations. High heterozygosity levels of all populations, studied (72-86%) indicate the usefulness of this polymorphism in population and genome fingerprinting studies.