To study a role of leukotrienes (LTs) in bronchial asthma, we measured LTC4 and LTD4 in plasma of 16 asthmatics during wheezing attack and of 4 normal subjects. Plasma of arterial blood was processed by extraction using the SEP-PAK, followed by high-performance liquid chromatography (HPLC). Fractions from HPLC with elution times which corresponded to synthetic LTC4 and LTD4 were used for quantification of LTs by radioimmunoassay (RIA) and bioassay. LTC4 was detected in 13 out of 16 asthmatics by RIA (mean +/- SD, 410 +/- 630 pg/ml) and in 5 by bioassay. LTD4 was detected in 10 out of 16 asthmatics by RIA (mean +/- SD, 540 +/- 290 pg/ml) and in 7 by bioassay. No LTs were detected by either method in the normal subjects. The present study shows that LTC4 and LTD4 play a role as chemical mediators during asthmatic attack.