Pollen of birch(Betula)and grass (Poaceae) are the main cause of seasonal allergy in northern European countries. Allergic symptoms may also occur outside the local pollen season. From the point of view of allergic sufferers, it is important to know how long pollen retains its sensitising power. The maintenance of antigenic activity under different experimental conditions was studied over a period of one year. Pollen of birch(Betula pendulaRoth.,B. pubescensEhrh.) and timothy(Phleum pratenseL.) grass was collected in the summer of 1995 in south‐western Finland. Pollen samples were incubated in petri dishes in separate desiccators, at 20°C at relative humidities of 23%, 33%, 43%, and 54%, which coincides with typical values in Finnish homes. Control pollen samples were stored under room conditions. The samples were analysed with igG‐ELISA (enzyme‐linked immunosorbent assay) using monoclonal (specific to Bet ν 1) and polyclonal birch pollen antibodies and Phl p 5 antibody, with SDS‐PAGE (sodium dodecyl sulphate‐polyacrylamide gel electrophoresis) and IgE‐immunoblotting using sera from birch pollen allergic subjects. Birch and grass pollen antigenic activity remained high in all treatments during the entire study period. Different relative humidities had no detectable effect on the antigen concentrations.