The efficacy of the Racal Airstream helmet respirator in preventing symptoms due to Laboratory Animal Allergy (LAA) was assessed in ten patients. Eight of these were established cases of asthma and two had severe rhinitis. Peak expiratory flow rate (PEFR) readings, recorded every two hours, were kept for seven weeks (six in exposure), together with a diary of subjective symptoms. Objective evidence of good protection was obtained in six out of the eight asthmatic patients; overt asthma was seen in the other two. The helmet respirator would appear to be a valuable adjunct in the management of occupational asthma in those who opt to remain in exposure. Those asthmatics who use a helmet respirator need to be monitored carefully and regularly to ensure that their respiratory function has not deteriorated. Persons with severe local symptoms of rhinitis and conjunctivitis also benefit subjectively from the use of the helmet although symptoms are not completely suppressed nor may progression towards asthma be prevented. The findings may well be applicable to the management of other types of occupational asthma but any inferences should be drawn with caution.