The Comet assay is a rapid and sensitive method for analyzing single cells for DNA damage. Using human lymphocytes, the assay is particularly useful for human monitoring studies, as well as for in vitro genotoxicity testing of chemicals. In such studies, it is not always possible to collect and process matched samples on the same day as the blood is taken. It would be useful if some samples could be stored and examined at a different time, without loss of viability or other factors affecting responses. It is thus important to understand the effects of storage conditions on blood to be used in such studies and how exposure or treatment might modify such responses. In a joint study in two laboratories, blood was taken from various donors and stored under different conditions. It was examined on day 1 (day on which sample was taken) and days 2, 3, 4, 5, or 8 at room temperature, 4°C, or -20°C. Cells were treated after storage (from day 2 onward) with bleomycin (BLM) and ethylnitrosourea (ENU). The data were analyzed either by eye (classifying cells with different categories of damage) or by using a computerized image analysis system (Kinetic Imaging Ltd., Liverpool UK, Software Package Comet 3.0) where the tail moment, which is considered to be a sensitive measurement, has been analyzed. There was no loss of cell viability at 4°C or room temperature up to 8 days when measured by trypan blue dye exclusion. Findings suggest that on days 1-4 for the untreated samples at room temperature or 4°C there were no biologically meaningful changes in both the different categories of cell damage and tail moment data. In treated cultures up to day 4, either at room temperature or at 4°C, responses were only minimally affected and changes were considered not to be of biological significance. However, there was slightly less variability between samples at 4°C than at room temperature in one laboratory. The reverse was true in the other. This would suggest that samples can probably be stored up to day 4 at 4°C or room temperature without any untoward effects. Provided samples can be processed within this 4-day time frame, it would not seem necessary to cryopreserve samples at -196°C.