The stability of tetrahydrocannabinol (THC) in whole blood during storage was investigated. Two types of containers were used: plastic tubes (polystyrene) and glass vials. Freshly drawn blood was spiked with 10 pmol THC/mL and stored for four days at room temperature (20 degrees to 25 degrees C). The material was kept for four weeks at -20 degrees C. Blood samples from 16 persons apprehended by the police on suspicion of cannabis smoking were collected into the two types of containers and stored frozen (-20 degrees C) until analysis. THC was quantified by capillary column gas chromatography/electron impact mass spectrometry (GC/EI-MS) combined with selected ion monitoring (SIM). Deuterium-labeled THC (THC-d3) was used as internal standard (IS) and was added to the samples immediately prior to extraction. The THC level remained unchanged in glass vials. Samples from the plastic containers had lost 60 to 100% of their THC during storage. In 13 authentic samples, the THC level in blood from the plastic containers varied 0 to 87% from the level in the same blood stored in glass vials. The IS was undetectable in some of these samples.