A case of solar urticaria is described showing: (I) Action spectra for late erythema (MED), late swelling and wealing with one peak of sensitivity for erythema and wealing at 405 nm. (2) No signs of porphyria. (3) Possibly increased skin mast cells. (4) Short-lived post-irradiation fibrin deposition. (5) Haemolysis. (6) Apparent suppression of urticaria with the antihistamine Incidal.