The generation of leukotrienes B4 (LTB4) and C4 (LTC4) by leucocytes stimulated with Ca ionophore A23187 was examined in 16 patients with asthma (8 with atopic and 8 with nonatopic asthma) and 12 healthy controls. 1. The LT84 generation by leucocytes was not significantly different between patients with asthma and healthy controls. The generation of LTC4 was significantly larger in patients with asthma than in healthy controls. The LTC4 generation was also significantly larger in patients with attacks (58.4 ± 38.5 ng/5x10(6) cells) than in those without attacks (23.3 ± 25.9 ng/5x10(6) cells)(p<0.05). 2.ln atopic asthma, the LTC4 production was significantly larger in patients with attacks (84.7 ± 35.4 ng/5x10(6) cells) compared to the production in those without attacks (40.4 ± 27.2 ng/5x10(6) cells)(p<0.02). However, the production of LTB4 was not significantly different between attack and attack-free stages. 3. In nonatopic asthma, the LTC4 production was also significantly higher in patients with attacks (32.2 ± 26.3 ng/5x10(6) cells) than in those without attacks (12.2 ± 3.5 ng/5x10(6) cells)(p<0.05). However, the LTB4 production was not significantly different between attack and nonattack stages. 4. The LTC4 production was significantly larger in atopic asthmatics compared to the production in nonatopic subjects both in attack and nonattack stages. These results suggest that the generation of LTC4 by leucocytes of patients with asthma is closely related to IgE-mediated reaction and asthma attacks.