Objective. Microorganisms such as streptococcus and autoantigens such as 60 kD heat-shock protein (HSP60) are implicated in the etiopathogenesis of Behcet's disease (BD). Methods. Peripheral blood mononuclear cells from patients with BD (n = 16) and healthy controls (HC) (n = 11) were cultured for 5 days with extracts of S. sanguis-KTH-1 (SS), E. coli (EC) and a mixed peptide combination from human HSP60 (aa 136-50, 179-97, 224-58 and 336-51) reported to be associated with BD. T and NK cell subset changes were determined by flow cytometry. Results. In unstimulated 5-day cultures γδ+ (both CD4+γδ+ and CD8+γ δ+), CD8+αβ+, CD4+CD56+ and CD8+CD-11b+ cells were increased in BD compared to HC. In antigen-stimulated cultures of BD patients CD3+ and αβ+ T cells responded to HSP60 peptides whereas EC stimulated only CD16/CD56+ NK cells. In the control group, similar to BD, αβ+ and CD4+ T cells responded to HSP60 peptides, however SS and EC mainly activated cytotoxic T cell subsets (CD8+CD11b and CD4+-CD56+ T cells). Conclusion. Significant increases in unstimulated T cell subsets suggest the presence of an in vivo T cell activation in BD. In both patients and controls similar patterns of responses were observed against different microorganisms, however the role of human HSP-60 peptides as immunodominant, cross-reactive antigens could not be demonstrated. © Copyright Clinical and Experimental Rheumatology 2004.