Blood samples were collected from EPIC participants at recruitment and these are stored at very low temperature (around -196 º C) in liquid nitrogen until they are required for laboratory analysis. Samples were obtained from virtually 100% of participants in each centre except in France and Oxford where, for logistical purposes, 33% were obtained. This facility for long-term storage enables comparisons to be made years later between results of laboratory measurements on samples from subjects who do and those who do not develop cancer during the follow-up period. The process for collecting and storing the blood was standardized throughout the 7 original participating countries. 30ml of blood was obtained and stored as plasma, serum, buffy coat and erythrocytes in 0.5ml cryo-plastic straws (CBS straws^TM). The straws, storage system and special straw-filling machines were developed for this project in the years leading up to the start of recruitment (in 1993) by CryoBio System and are now being used in several large research studies in addition to EPIC.

Denmark, Sweden and Norway joined EPIC a little after the other centres. Similar, but not identical, blood storage systems were used in Denmark and Sweden, and in Norway blood samples are being collected and stored using the original EPIC protocol for plasma, buffy coat and erythrocytes.

The blood samples are an extremely valuable resource. In order to optimize security half of the sample is stored locally to each centre and half is stored at IARC.

There are several types of laboratory analyses which can be used to investigate the relationship between diet and cancer, including markers of diet (for example, levels of vitamins), markers of diet-related factors (for example, indicators of antioxidant status), markers of hormones which themselves can be influenced by diet and measurement of the 'fingerprint' (adducts) left by natural or chemical contaminants on DNA.