Of thirty-eight children, who were immunized with mould extracts, seven (19%) were withdrawn from hyposensitization due to serious side-effects clinically consistent with a type III reaction. These seven children exhibited a two- to four-fold increase in circulating precipitating-antibodies to the injected extracts, and had shown a slight increase in precipitating antibodies to the mould extracts used before hyposensitization. A further fourteen patients, also hyposensitized with mould extracts but without side effects did not develop precipitating antibodies. It is suggested that hyposensitization with mould extracts may be hazardous in children who may have a primary "immune imbalance' with a risk of provoking type III reactions.