The reaginic activity in serum from pollen-allergic children, determined by the radioallergosorbent test (RAST) was followed before, during and after the pollen season. In addition, the serum concentration of IgE was examined. The investigation included both untreated children and children treated by inhalations of disodium cromoglycate for pollen-induced asthma. Typical seasonal variations of IgE antibodies and pollen allergens, as well as total IgE, were found in the majority of the children. In general, changes of specific antibodies were considerably more pronounced than those of total IgE. The serological course was characterised by a rapid increase in reaginic activity during the pollen season. In some patients reagins could not be determined by the RAST technique before the season. However, reagins were found both during and after the season. In 1 patient, circulating reagins were found during, but not after, the pollen season. Very favourable results were obtained following treatment with disodium cromoglycate. Treated patients showed pronounced serological changes of a type similar to those observed in untreated cases, despite absent or insignificant asthma symptoms. In a group of patients, allergic to birch pollen, that was treated with disodium cromoglycate the average increase in the birch reaginic serum level was almost twice as high as that found in untreated patients allergic to birch pollen.