In view of great species differences in biology of polymorphonuclear cells, and non-availability of basic data on buffalo PMN cells for assessing their functional activity, the present work on the immuno-defence system involving protein synthesis and O2- production was undertaken to highlight the immunomodulatory role of thyroxine. Digitonin, LPS and Con-A activation generated superoxide, which was monitored by NBT reduction. The study suggested that concanvalin A (Con-A) and T4 were able to synergetically increase the production of superoxide and H2O2. The likely involvement of thyroxine in activation was studied by [125I]thyroxine incorporation, which was significantly increased due to activation. In contrast, aflatoxin B1 together with Con-A caused a significant decrease (P < 0.05) in incorporation of [125I]T4. Optimum time dependence in [14C]leucine incorporation by buffalo PMN cells was found to be 30 min and the factors like T4 (7.7 ng/ml) and glutathione (400 micrograms/ml) significantly enhanced the incorporation. In contrast, antiinflammatory agent, indomethacin (40 micrograms/ml) inhibited protein synthesis in PMN cells; while puramycin also significantly lowered the [14C]leucine incorporation. Total [14C]leucine incorporation in acid extractable cationic proteins and peptides, known for their antibacterial properties was found to be 30-40% when separated on AU-PAGE. The studies revealed the in vitro immunomodulatory role of T4 in O2-, H2O2 production and cationic protein synthesis by the activated PMN cells of buffalos.