The number, distribution, and phenotype of mononuclear cells infiltrating the allograft lung transplant were determined immunohistochemically with monoclonal antibodies directed against cellular antigens (CD3, CD4, CDS, CD22, CD25, CD 16, CD56, CD68, HLA-DR) on frozen sections of transbronchial biopsies. Seventy-two transbronchial biopsies from 21 patients undergoing lung or heart-lung transplantation were evaluated histologically and immunohistologically in a prospective study. Four major results were obtained in the graft lung parenchyma: (1) whatever the histological grading of rejection, T lymphocytes expressing CDS were present and in a significantly higher number than in control subjects (P+, CD8+, CD25+, CD16+ cells (P<0.01); (3) the CD4/CD8 ratio was inverted (0.52 ±0.04), with no correlation with the histological rejection; and (4) the number and