Arterial and venous whole blood and plasma histamine concentrations and eosinophil and basophil counts were determined in 5 patients with acute severe asthma who had not previously received steroid therapy, in 5 who had been maintained on steroid therapy and in a control group of 9 patients with acute non-respiratory illnesses. No significant arteriovenous differences were observed for any of these measurements in any of the groups of patients. Significant net loss of arterial histamine does not occur peripherally in acute asthma. When compared with the values for the controls, statistically significant increases were observed, in the group not receiving steroids, for arterial and venous whole blood histamine concentrations, eosinophil counts and basophil counts and, in those receiving steroids, for the venous basophil counts. When compared with the venous plasma histamine concentration of normal subjects, that of the asthmatic subjects not receiving steroids was significantly raised. The venous plasma histamine concentration of the control group was also significantly higher than that of normal subjects, but less so than in the asthmatic subjects, suggesting that acute illness per se produces an increased plasma histamine concentration. Both groups of asthmatic patients were treated similarly with hydrocortisone and bronchodilators. There was a striking fall in whole blood histamine concentration and in eosinophil and basophil counts, but plasma histamine fell more slowly, especially in those who had not previously received steroids. The mean histamine content of the basophil cell is 0.01 pmol and significant differences in this value did not occur within the various groups or as a result of treatment. The approximate number of molecules of histamine per basophil cell is 6.0 .times. 109.