This study investigates the early humoral immune response following natural exposure to an inhalant antigen (Dermatophagoides pteronyssinus) in 36 babies, from birth until 1 year of age. The total IgG and subclass 1 and 4 D. pteronyssinus-specific antibody levels were assayed in sera collected at 7 days, 3 and 12 months by ELISA. After an initial fall, due to the progressive loss of maternal antibodies, an IgG specific response to D. pteronyssinus was seen between 3 and 12 months. This was restricted to the IgG1 subclass when the values at 12 months were significantly higher than those detected at the third month (P less than 0.001, paired t test). No D. pteronyssinus-specific IgG4 antibody was detected in any subject at any of the time points tested. The present study demonstrates that inhalant as well as food antigens are able to stimulate the immune system during the first year of life and that the antibodies produced are of the IgG1 subclass.