The basophilic propagating activity was determined in conditioned media obtained from cell cultures of mononuclear cells from atopic and healthy individuals. The activity was analyzed as the capacity to induce differentiation in the human basophilic cell line KU812. The KU812 cells responded primarily to cultured media with histamine production and secondarily with granulation. Eight atopic individuals, 3 of whom having mild symptoms, with known birch allergies were selected together with 3 control individuals. Total and specific IgE were analyzed in sera, and basophil and eosinophil counts were determined from blood smears after Wright's staining. Significant differences between the symptomatic atopic individuals and the control group were obtained for eosinophil counts (p less than 0.05) and in the KU812 basophilic differentiation activity assay (p less than 0.01). No single test could significantly discriminate between symptomatic and nonsymptomatic individuals, but the KU812 cell assay in combination with the eosinophil count clearly distinguished the symptomatic individuals from the control group. Increased levels of IgE specific to birch allergen were only obtained in the atopic individuals (p less than 0.001). No difference in basophilic propagating activity was observed with supernatants from cells incubated with birch pollen allergen. Serum tested for basophilic propagating activity showed no or decreased values of inducing histamine production in the KU812 cell line. In conclusion, the KU812 assay for basophilic propagating activity was the most useful discriminating test to select symptomatic atopic individuals from nonsymptomatic atopic and control individuals.