Hydrogen cyanide and carboxyhaemoglobin were not found in the blood samples taken from corpses of victims who were set on fire after death (homicide, suicide, accident). In the control group (postmortem blood samples from persons suspected of carbon monoxide poisoning), the presence of blood HCN was found in only 8 out of 135 examined blood samples. The concentration was approximately 0.1 mg/ml, which was below the limit of detection assumed in the study. An increase in postmortem endogenous hydrogen cyanide concentration reached a level of significance for forensic medical expert opinions only after 2 weeks of storage of the blood samples at a refrigerator temperature of +4 degrees C. The study concludes that the threat of adulteration of postmortem blood samples due to endogenous hydrogen cyanide during storage is slight. The study used 167 postmortem blood samples that were separated into 3 groups. One group consisted of samples of nonputrefied blood taken from 20 dead bodies that were initially found to be negative for the presence of HCN. These samples were stored at 4 degrees C and examined for the presence of HCN every 7 days for 12 weeks. A second group consisted of blood samples from corpses of 12 persons found in fires; the blood samples were analyzed immediately after collection of samples. The third group consisted of 135 blood samples with signs of postmortem autolysis that were taken from corpses of persons poisoned by carbon monoxide not connected with fires. Analysis of these samples was conducted immediately after collecting the samples. 3 tables, 2 figures, and 17 references