Sixty children, 7--15 years old, suffering from bronchial asthma and 44 healthy children, 12--14 years old, were studied for the percentage and the total count per mm.3 of T and B lymphocytes. Patients were classified into extrinsic atopic, mixed and intrinsic asthma. According to the clinical course mild and severe cases were analysed separately. The total and "active" T lymphocytes were detected by the ability to form nonimmune rosettes with sheep red blood cells. The B lymphocytes were estimated by the rosetting procedure with antibody mouse complement coated sheep erythrocytes. No decrease of the percentage or total count of T lymphocytes was observed, and an elevation of total count and percentage of active T cell rosettes was even seen in severe cases of extrinsic atopic asthma. The B cell percentage was decreased in severe atopic, mild intrinsic and mild mixed asthma but the total number was lowered only in the last group. The immunoglobulin A, G, M, D and E level, measured by the single radial immunodifussion test, was correlated neither to T cell nor B cell percentage. The complement components C1q, C4, C3, C3 proactivator and C5 were measured by the radial immunodufusion test in the plasma of some patients and controls. The C1q level was normal. An elevation of C4 in mild extrinsic, C5 in severe extrinsic and C3 in severe mixed asthma was seen. A decrease of C3 proactivator was observed in mild extrinsic and mixed asthma, and in three cases of intinsic asthma this was accompanied by lowering of C3, suggesting an activation of the alternative pathway.