Pertussis inoculated mice have an enhanced susceptibility to lethal effects of water soluble extract of rye grass pollen (WSR). Five days after the administration of Bordetella''[Haemophilus] pertussis vaccine, 50% of the inoculated mice survived intravenous injection of 5 mg of WSR and only 18% survived 20 mg of this material. Normal mice suffered no fatalities from equivalent challenge doses of WSR. A mixture of WSR with specific immune, or normal, rabbit serum killed more pertussis inoculated mice than pollen extract alone.