The influence of storage time and storage temperature on the yield of chromosomal aberrations after in vitro exposure of blood lymphocytes has been investigated. Heparinized venous blood was stored at 5°, 20°, or 37°C for periods of 0, 24, 48, or 173 hr and was then given 0, 125, 250, or 500 R of x-irradiation. Immediately after exposure, the blood was cultured under standard conditions for 48 hr. Metaphases were examined for the presence of polycentrics, rings, minutes, chromatid exchanges, and breaks. Storage for 24 hr, particularly at higher temperatures, increases significantly the yield of aberrations. Storage at 5°C for 173 hr doubles the yield of radiation-induced aberrations but has no influence on aberrations present in nonirradiated blood.