1. Histamine levels in whole blood and blood plasma, together with circulating basophil, eosinophil and total leucocyte counts, were determined in three groups of asthmatic children and in a control group of non-asthmatic children. 2. Blood histamine levels were significantly higher than the control levels during asthmatic symptoms, but fell to near normal values in the quiescent phase of asthma and during long-term steroid treatment. 3. Plasma histamine levels were found to be minimal in all the subjects studied (< 1 ng/ml). 4. Significant correlations were found between whole blood histamine levels and basophil counts and between whole blood histamine levels and log eosinophil counts in the control children. The findings in the asthmatic children suggested that the relationships between these phenomena in asthma are more complex than in normal individuals.