Immunological functions of the pharyngeal tonsil, palatine tonsils and blood leucocytes of children undergoing tonsillectomy were evaluated by determining T or B lymphocytes, the response to mitogens, and the cell-mediated immunological responses to tuberculin. In all the test systems used similar results were obtained with cells derived from either the palatine or pharyngeal tonsils. The mean percentage of T lymphocytes was significantly higher in the peripheral blood than in tonsils, but the reverse was true of B lymphocytes. The reaction to PHA was lower in tonsillar cell culture than in blood cell culture, but tonsillar cells reacted better to Con A than blood cells. In lymphocyte transformation tests tonsillar cells reacted to specific antigen (tuberculin) and this reaction was significantly higher than that of the parallelly tested blood lymphocytes. Further, in about 50% of the children tested, tuberculin caused migration inhibition of the mixture containing tonsillar cells and guinea pig peritoneal cells. Surprisingly, nearly identical results were obtained if migration inhibition test was performed with tonsillar cells alone. Consequently, poorly migrating tonsillar cells are nevertheless usable for direct migration inhibition testing.