Histamine and sera complement were followed-up in patients with chronic urticaria during oral provoking test to cow milk. Three patients with a positive history to cow milk sensitisation showed a significant decrease in C3 concentration within 180 minutes of the test. This was followed by increased histamine and skin eruption. According to our results (skin test reactivity and clinical evaluation of the provoking test) several mechanisms were involved in the sensitisation to cow milk in our patients.