SummaryLocal exudation at the sites of allergic and histamine‐induced cutaneous reactions, was measured with anin vivodouble‐labelling procedure, based upon local serial simultaneous measurements of indium‐113 m (113mIn) labelled transferrin and technitium‐99m labelled red blood cells (99mTc RBC). Analyses depended upon the reaction index i.e. comparison of the ratio of113mIn/99mTc counts at the reaction site with the ratio of113mIn/99mTc at the corresponding unreacted site in the skin of the opposite arm.Reaction indices were initially significantly elevated at the sites of histamine and allergic reactions. After the first hour, there was a progressive reduction at histamine sites indicating a tapering of the local accumulation of indium‐labelled transferrin. By contrast allergic reaction sites showed a continuing increase in the reaction index after one hour. During the first hour the curves were comparable, but they diverge and were significantly different after one hour (0.05>P>0.02). This is consistent with the known occurrence of late cutaneous responses at the site of allergic but not histamine