The authors present the results of VNTR-PCR DNA polymorphism of three STR loci. Gene profiles were determined in experimental spots of blood from 2 or 3 unrelated individuals mixed in the ratios 1:1-1:100. Genotypes were successfully determined in mixed blood spot samples of ratios 1:1-1:60. A small amount of DNA in all those samples manifested a light electrophoretic staining. The electrophoretic band disappearance in the mixed blood spot samples of high dilution (<1:70) did not allow for determination of their genotypes. In the paper the results obtained while identifying biological traces from sexual offences in which mixed DNA occurred have been also presented.