Phenolic glycolipid-I (PGL-I) is known to be a major antigen of Mycobacterium leprae. We have studied the influence of PGL-I on the production of Tumour Necrosis Factor alpha (TNF-alpha) using the in vitro whole blood assay. Armadillo-derived M. leprae (ADML) are thought to be depleted of PGL-I during the purification process. M. leprae obtained from mouse foot pad material (MFPML) has been subjected to a less rigorous purification process; their PGL-I coating is therefore believed to be more intact than that of ADML. PGL-I or ADML alone induced the secretion of minimal levels of TNF-alpha in whole blood assay; when added in combination, higher levels of this cytokine were observed. The highest TNF-alpha response was seen following stimulation with MFPML. MFP material not infected with ML did not elicit any response. The difference in TNF-alpha response shown by ADML and MFPML was postulated to be largely due to the presence of higher levels of PGL-I in MFPML. This increase in TNF-alpha production suggests that PGL-I may play a significant role in the induction of TNF-alpha during natural infection.