The Biologist in forensic work is confronted with many problems on grouping of dry blood stains, these problems are due to many factors and environmental changes which may mislead the analysis. The present work deals with the factors affecting ABO grouping of dry blood stains in Riyad, including exposure to extremes of temperature, from refrigeration at -4 degrees C up to heating at 150 degrees C, effect of time till 6 months, occurrence of the stains on different fabrics, and effect of putrefaction. The results showed that, ABO grouping of dry blood stains was not affected by different substrata or cloth fabrics. 30% of the samples gave erroneous results when exposed to temperature at 150 degrees C for 2 hours. Storage for 6 months at room temperature ranged from 29 degrees -44 degrees C, representing the ambient temperature met with in Riyad, gave no effects. Putrefaction showed that the antigens were deteriorated in 80% of cases after 90 days. Chromatographic separation was a suitable procedure before grouping the samples which were buried in sand or mud.