Eosinophil cationic protein (ECP) and ECP/eosinophil ratio were measured in 223 apparently healthy subjects. The serum sample for ECP measurement was obtained by standardized clotting of the blood sample for 2 hours at 37 degrees C. Eosinophil count, ECP concentration and ECP/eosinophil ratio were no different between men (n = 122) and women (n = 101). Reference ranges for serum ECP and ECP/eosinophil ratio were 12-99 micrograms/L and 61-367 micrograms ECP per 10(9) eosinophils, respectively. Serum ECP and blood eosinophil count were positively correlated (y = 141x + 18, R2 = 0.45, P < 0.001). The ECP/eosinophil ratio, however, was found to decrease with increasing eosinophil count. Twenty-three per cent of the apparently healthy subjects were found to have a positive score for immunoglobulin E antibodies specific to inhalant allergens, and thus were considered atopic. Serum ECP concentrations and eosinophil counts were significantly higher in this group compared with the non-atopic group. In this healthy population no decision level for either eosinophil count, serum ECP or ECP/eosinophil ratio could be found that discriminated atopic from non-atopic individuals.