The histamine content in plasma samples obtained from a group of healthy blood donors, from blood stored in citrate-phosphate-dextrose supplemented with adenine (CPDA) packs for up to 28 days, and from the side arm of a transfusion line was measured by radio-enzymatic assay. Healthy donors had a mean plasma histamine content of 0.79 ng per ml. Blood stored in CPDA initially showed a similar histamine level (0.69 ng/ml on day 3 of storage), but there was a progressive rise with time, and at 28 days, the level was 20.5 ng per ml. The increase in histamine is best described by a positive exponential and may be explained by a process whereby the plasma histamine level increases the degree of histamine release from blood cells. The histamine levels in the blood infused into patients tended to be higher than those found in the stored units of the same age, if these packs were less than 7 days old. This may have been caused by the unit becoming warmer during transfusion. We speculate that the histamine levels in the older units of stored blood were high enough to cause or augment transfusion reactions and that the storage age of blood may have a bearing on the incidence of transfusion reactions.