The AB0 blood group system typing remains one of the basic laboratory tasks in a forensic practice. However, problems arise when the analysed samples are seriously degraded. We took blood samples from six volunteers (three men, three women) and made blood stains on pieces of sterile cotton cloth. Blood stains were incubated at three different temperatures (22 degrees C, 37 degrees C, 56 degrees C) for various periods of time (1 day, 1 week, 14 days, 1 month, 3 months, 6 months, 1 year). For blood stains degraded at 22 degrees C we also analysed the samples after 3.5 hours of incubation. Moreover, we tried to determine the AB0 blood group system after thermal degradation at high temperature, accurately at 200 degrees C for 10 min. For the AB0 blood group system typing a Polymerase Chain Reaction method was used to amplify glycosyltransferase gene, when DNA had been isolated from artificially created blood stains, followed by their subsequent artificial thermal degradation. For serological AB0 typing the mixed agglutination and the Therkelsen method were used. The DNA analysis seemed to solve problems with seriously degraded blood stains but we found out that classical serological methods were even better in some cases.