Increasing evidence implicates effector T-cells in the pathogenesis of hypersensitivity pneumonitis. We utilized T-cell- and Ia-specific antibodies, flow cytometry, and computer analysis to quantitate T-cell numbers and Ia expression on lung cells of established rabbit models of acute and chronic hypersensitivity pneumonitis (HSP). We used analysis of variance (ANOVA) and Turkey's follow-up to evaluate group differences. In the acute HSP group, increased percentages of T-cells and greater Ia expression were present on bronchoalveolar lavage (BAL) cells at 24 h after inhalational allergen challenge. Increased BAL T-cell numbers were found in the chronic HSP group produced by repeated inhalation of antigen and muramyl dipeptide, compared to "desensitized" animals and control groups. Both chronic HSP and desensitized groups demonstrated increased Ia expression on BAL cells. Pathology scores for individual animals in both acute and chronic protocols correlated significantly (Pearson correlation coefficients) with total numbers of BAL cells, percentages of T-cells, and percentages of Ia-positive cells recovered. Findings are consistent with the hypothesis that cell-mediated hypersensitivity is a central mechanism in the pathogenesis of experimental hypersensitivity pneumonitis.