Seven of twelve autologous sera from patients with chronic idiopathic urticaria re-injected intradermally produced a weal at the site of injection. There was no response in 19 control subjects. Patients showing a positive response had a shorter duration of disease and shorter duration of spontaneous weals, and their urticaria was less likely to be exacerbated by pressure. There was some serological evidence of circulating immune complexes in both positive and negative responders to autologous serum, but only two showed complement abnormalities. When six of the serum-positive patients were re-tested after one year, five still showed a positive response with their original stored serum, but only two, whose disease remained active, were positive when challenged with freshly drawn serum, suggesting that a serum mediator is only present when the urticaria is active. A marked neutrophil infiltrate was seen within and around small dermal blood vessels at the injection site in the majority of urticaria patients but this appearance did not correlate with weal formation. In control subjects the cellular response was mild and mainly mononuclear.