Rationale: M. avium and M. abscessus are ubiquitous environmental organisms increasingly recognized to cause chronic lung disease in patients with apparently normal immune function. Little is yet known about their human pathophysiology. Objectives and Methods: To examine cytokine, chemokine responses (protein and gene expression) and signaling pathways triggered by reference and clinical isolates of M. abscessus and M. avium in human peripheral blood mononuclear cells, monocytes and murine bone marrow-derived macrophages in vitro. Results: M. abscessus-induced tumor necrosis factor alpha (TNF)production was higher than that induced by M. avium. Interferon-, interleukin-1, and the chemokines macrophage inflammatory protein-1 and regulated on activation, normal T cell expressed and secreted (RANTES) were equally up-regulated. Differences between MAB and MAV do not require replication and are heat stable. We found no differential effect due to rough or smooth colonies within the same species. Similar to M. avium, M. abscessus triggered mitogen activated protein kinase (MAPK) signaling and nuclear factor-B translocation. Induction of TNF was dependent on MAPK pathways since pre-incubation of cells with signaling inhibitors led to >85% reduction in cytokine secretion. M. abscessus also triggered a Toll-like receptor 2 (TLR2)-mediated response that led to TNF production by human monocytes. Accordingly, stimulation of murine TLR2- or myeloid differentiation factor 88-deficient bone marrow-derived macrophages did not elicit TNF, reinforcing a critical role for TLR2 in M. abscessus-induced cell activation. Conclusion: M. abscessus signals human cells through MAPK and TLR2 pathways and triggers more pronounced pro-inflammatory cytokines and chemokines than M. avium.