The finding of peanut agglutinin (PNA) binding capacity, supposed to be a marker 
of immature lymphocytes, within the T-cell population of the human thymus (58%) 
and tonsil (10%) prompted the comparison of maturation stages of PNA binding 
(PNA+) and nonbinding (PNA-) T cells in both organs. The proliferative response 
after mitogenic stimulation of purified PNA+ fractions was significantly less 
than that of purified PNA- fractions. The results of mitogen dose-response 
experiments, of variation in time of culture harvest, and of addition of 
irradiated allogeneic peripheral blood non-T cells indicated the intrinsic 
mitogen unresponsiveness of cells in the PNA+ fractions. The mitogen response of 
tonsil fractions was higher than that of thymocyte fractions. Cells with an 
immature immunologic phenotype were enriched in the thymocyte PNA+ fraction, and 
almost absent in the tonsil fractions. Both tonsil fractions contained cells 
with the immunologic phenotype of mature T cells, and showed a purine 
interconversion enzyme makeup comparable to mature T lymphocytes. It is 
concluded that the tonsil PNA+ T cell is a functionally immature lymphocyte 
which is in a further maturation stage than PNA+ or PNA- thymocytes. The 
presence of PNA+ T cells outside the thymus is of relevance for the clinical 
evaluation of PNA binding assays and suggests the occurrence of T-cell 
maturation within the tonsil environment.