Apoptosis, a programmed cell death results from interaction between surface molecule Fas and its ligand FasL. Fas expression is increased on activated T cells. FasL is upregulated following TCR stimulation. The aim of our study was to investigate Fas and FasL expression following stimulation of PBMC with specific allergen. 17 allergic subjects, sensitive to grass pollen and 15 healthy controls were included in the study. PBMC were isolated by density gradient centrifugation and incubated with various concentrations (1, 10, 100 ng/ml) of allergen for 18 h at 4^oC. Standard immunofluorescence test was performed using rabbit IgG against human Fas and FasL. The expression of both Fas and FasL after stimulation with allergen was significantly increased in allergic subjects whereas in healthy ones incubation with allergen did not affect the expression of these molecules. Our results indicate that specific allergen stimulation of lymphocytes from allergic subjects might induce Fas-FasL apoptotic pathway, which might result in deletion of specific cells. This might be a possible mechanism of allergen tolerance achieved naturally or following specific immunotherapy.