This investigation examines the effect of a booster dose and cyclophosphamide treatment in guinea pigs sensitized according to different sensitization regimens. The results show that guinea pigs sensitized with a high dose of ovalbumin (10 microgram) develop a transient bronchial reactivity. The administration of a booster dose to guinea pigs sensitized according to this regimen does not result in any secondary response. Cyclophosphamide 30 mg/kg given 2 days before the booster injection or the primary sensitization results in a marked increase in bronchial reactivity when challenged with ovalbumin. This increased bronchial reactivity is reflected in the IgE-like antibody titre examined by PCA. Guinea pigs primarily sensitized with a low dose of ovalbumin (0.5 microgram) show no bronchial reactivity when challenged with antigen. However, guinea pigs sensitized according to this regimen and given a booster dose show a marked secondary response when challenged 7 days after the booster injection. An examination of the antibody classes by PCA show that the booster injection causes an increase in the IgG1--and also in the IgE-like antibody production.