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Left lung transplantation (n = 30) between BN rat (RT1ⁿ) and LEW rat (RT1¹) was performed to examine serial changes in the inflammatory cell profile and T-cell subsets occurring in bronchoalveolar lavage (BAL) obtained after transplantation. Transplanted animals were sacrificed on Days 1 to 7 post-transplantation. Previous studies show that lung allografts between these rat strains were strongly rejected within 7 days. The serial change in the cell profile of BAL showed a marked initial predominance of polymorphonuclear leukocytes, a decrease in macrophages, and a temporary increase in number of eosinophils on Day 2 posttransplantation. A gradual increase in lymphocytes coincident with progression of rejection was also noted. Immunocytologic studies using monoclonal antibodies specific for rat T-cell subsets and interleukin-2 receptor (IL-2R) showed significant increase in pan-T-cells on Days 3, 4, and 5 and T-suppressor/cytotoxic (CD8 positive) fraction on Days 4 and 5, whereas the T-helper (CD4 positive) fraction peaked on Day 2. The frequency of T-cells expressing IL-2R (55 kDa), indicating activated T-cells, significantly increased as early as Day 2 and maintained its high value thereafter. mRNA levels for IL-2R were detectable in the allografts on Day 2 and peaked on Day 5 post-transplantation. The value of CD4/CD8 T-cell ratios rose initially and then dropped below 1.0 on Days 4 and 5. These values differed markedly from those of syngeneic transplants (Lew → Lew) examined on Day 5 post-transplantation. First, no significant changes in BAL cytology were seen when syngeneic transplants were compared with normal (Lew) lung. However, there were significant differences between syngeneic and allogeneic transplants preformed on Days 1-5 (P < 0.01). All immunocytology parameters (CD4-, CD8-, and IL-2R-positive T-cells) were significantly (P < 0.01) different between syngeneic and allogeneic transplants at each time point examined. No IL-2R mRNA was detectable in syngeneic transplants on Day 5, and none was detected in normal lungs. In summary, the following cellular events in BAL were felt to correlate closely with the histological diagnosis of rat lung allograft rejection: (i) An increase infiltration of T-cells bearing the pan-T-cell marker, OX-19, (ii) an increase in T-cells hearing the CD8 phenotype with a progressive decrease in the CD4/CD8 ratio, (iii) a progressive increased accumulation of mRNA specific for IL-2R and increased expression of IL-2R on graft-infiltrating T cells, (iv) an increase in the number of eosinophils infiltrating the graft. Copyright 1993, 1999 Academic Press