Cytokine-mediated interactions between monocytes/macrophages, lymphocytes, and eosinophils may be important in regulation of airway inflammation in asthma. Peripheral blood cytokine concentrations were measured in twenty adults with severe, acute asthma, ten with mild asthma (intermittent inhaled beta-agonists), twelve with chronic asthma (high-dose inhaled steroids), and in sixteen healthy subjects. Interleukin-1 beta, tumour necrosis factor, and interferon gamma were not detected in most subjects. Granulocyte macrophage colony-stimulating factor concentrations were higher in patients with severe asthma than in healthy subjects (median 179 vs 80 pg/ml; 95% confidence interval for difference 16-168; p = 0.009). Plasma concentrations of soluble interleukin-2 receptor (sIL-2R) were similar in all asthma groups (geometric mean in severe acute group 570 U/ml, mild group 559 U/ml, and chronic group 560 U/ml) and higher than those in healthy controls (361 U/ml, p less than 0.01). Sequential sIL-2R measurements were done in fifteen of the patients with severe acute asthma. There was no difference between baseline concentrations and those after 2 days of steroid treatment but by day 7 they had fallen significantly (ratio day 7/day 3 = 0.74, [95% CI 0.67-0.82]; p less than 0.001); however, there was no correlation with improvement in peak expiratory flow. This study provides further evidence for T-lymphocyte activation in asthma but shows that plasma sIL-2R is not a useful marker of disease activity.