To determine the variation of antigenic water soluble proteins in white birch (Betula pendula) pollen, extracts of pollen from different sides of individual trees were analyzed by isoelectric focusing (IEF), crossed immunoelectrophoresis, and crossed radioimmunoelectrophoresis. IgE-antibody-binding patterns were also studied in samples analyzed by IEF by probing with serum pooled from patients with birch pollen allergy, followed by radiolabelled anti-IgE. Antigenic proteins and allergens per unit weight of extracted protein were greatest in pollen extracts from the south side of the trees. Allergens decreased progressively in pollen from west- through east- to north- facing branches. Proteins with high isoelectric points (pI > 8.5) and proteins between pI 4.5 and 5.6 were infrequent in extracts from the north side. Extracts from branches facing north were poor in allergens: in general, only one or two precipitin lines were found, and in some cases they did not bind to IgE antibodies. Differences between numbers of proteins and allergens found in extracts from south, west and north branches were statistically significant for all methods used. The results indicate the need to collect birch pollen for allergen extract manufacture from south-facing branches.