The respiration characteristics and energy metabolism of sheep lymphocytes collected from the popliteal lymph before and after antigenic challenge with Salmonella organisms have been studied at various O2 tensions. The O2 consumption of sheep lymphocytes could be related to the PO2 of the medium by the Michaelis-Menten equation. In resting lymphocytes, Vmax (mean +/- S.D.) was 1.70 +/- 0.23 mumoles/h/2 X 10(8) cells), critical O2 concentration [( O2]crit) was 2.00 +/- 0.20 microM and Km was 0.46 +/- 0.11 microM. Significant changes occurred in the respiration characteristics of these cells following antigenic challenge of the node. Both Vmax and Km reached maxima when the blast cell response in the lymph was at its peak. [O2]crit, however, did not change significantly over an immune response. Lymphocytes collected before antigenic challenge had mean rates of glucose uptake of 0.97 +/- 0.29 mumoles/h/2 X 10(8) cells and lactate production of 1.31 +/- 0.37 mumoles/h/2 X 10(8) cells. During the height of immune responses to Salmonella, lymphocytes showed an enhanced glucose uptake, lactate production and C-1- and C-6-labelled glucose metabolism. The rates of energy metabolism of sheep lymphocytes varied with changes in the PO2 of the gas phase from 21% to 0% O2. The rates of glucose uptake and lactate production increased while the rates of 14CO2 production from C-1- and C-6-labelled glucose decreased as the PO2 decreased.