The number of antigen‐reactive cells among human peripheral blood T lymphocytes was estimated by a limiting dilution analysis. Antigen‐induced lymphocyte activation was measured by means of incorporation of tritiated thymidine [³H]dThd. We have studied the frequency of memory T cells for the bacterial antigens tuberculin PPD and tetanus toxoid in immune donors, as well as the frequency of alloantigen‐reactive T cells.

In 11 different donors, the observed frequencies of the antigen‐reactive T cell ranged between 1:300 and 1:16000 for PPD; for tetanus toxoid values, between 1:750 and 1:11500 were obtained in 5 different donors. The frequency of alloantigen‐reactive T cells was found to be higher: between 1:200 and 1:600 (n=10). For 3 donors, the estimated frequencies proved to be reproducible over a period of several months. Finally, a correlation could be demonstrated between the frequency of PPD‐reactive T cells and the [³H]dThd incorporation of 4 × 10⁴ PPD‐stimulated lymphocytes.