Thymidine incorporation in lymphocytes obtained from adenoids (AVL), tonsils (TL) and blood (PBL) and stimulated by PPD was compared in 19 BCG-vaccinated patients. Responses were approximately tenfold higher in vaccinated than in unvaccinated patients. The responses obtained for AVL/TL were lower than for PBL in BCG-vaccinated as well as in unvaccinated patients. In autologous mixtures of AVL/TL and PBL a linear increase in thymidine incorporation was obtained with increasing numbers of PBL. The monocyte concentration in cell suspensions obtained from adenoid and tonsil tissue was lower than in cells prepared from blood; addition of autologous monocytes purified from blood to cultures of AVL and TL was followed by a small increase in response to PPD, but not to the level of responsiveness of PBL. Addition of autologous T lymphocytes purified from blood to cultues of AVL and TL was followed by a small increase in thymidine incorporation, of similar proportions in cultures with and without PPD. It is concluded that the proportion of PPD-sensitized lymphocytes is smaller in AVL/TL than in PBL.