Guinea pigs inbred for their ability to develop respiratory anaphylaxis to experimental antigens have been used for comparison of different forms of immunotherapy (IT). Passive, active and combined (immune complexes between antigen and specific IgG) IT were compared with placebo. The bronchial reactivity of the animals to the antigen was monitored regularly before, during (35 weeks) and after IT (20 weeks). Animals treated with passive IT did not improve clinically. Active and combined IT abolished most symptoms within 7 weeks of treatment. During the post-treatment period, animals from both groups surprisingly recovered their original sensitivity to inhalation of the antigen.