The aims of the present study were to investigate the expression of Toll-like receptor (TLR)2 on the peripheral blood monocytes of patients with nontuberculous mycobacterial (NTM) lung disease and healthy controls, and to assess the responses of these monocytes to TLR2 agonists such as Mycobacterium avium and lipoteichoic acid (LTA).Reverse transcriptase-PCR was used to analyse TLR2 mRNA expression in peripheral blood monocytes from 17 NTM patients and 10 healthy controls. mRNA and protein secretion levels were also determined for the cytokines interleukin (IL)-12 p40 and tumour necrosis factor (TNF)-a.Expression of TLR2 mRNA by peripheral blood monocytes after stimulation with M. avium or LTA was lower in NTM patients than in healthy controls. IL-12 p40 and TNF-a mRNA and cytokine secretion levels were also lower in patients than in healthy controls. Treatment with anti-TLR antibody decreased M. avium-and LTA-induced IL-12 p40 and TNF-a production in healthy controls, but not in NTM patients.The present results suggest that the downregulation of Toll-like receptor 2 and the resulting decreased production of interleukin-12 p40 and tumour necrosis factor-a following Mycobacterium avium or lipoteichoic acid stimulation may contribute to host susceptibility to nontuberculous mycobacterial lung disease.