Materials and Methods.-Sensitization: The antigens used were ovalbumin (Worthington) and o-chlorobenzoyl chloride conjugated to bovine gamma globulin (OCBC-BGG) by the method of Benacerraf and Levine, 5 kindly supplied by Dr. Y. Borel. Guinea pigs of the Hartley strain weighing 500-800 gm were sensitized with the appropriate antigen diluted in saline and emulsified in an equal volumeof complete adjuvant (Difco H37Ra). Each animal received a total dose of 100, ug of antigen distributed into the four footpads, 0.1 ml per footpad. Tissue culture media: The basic tissue culture media used throughout were minimal essential media, Eagle 12-125 (Microbiological Associates, Bethesda, Md.) containing 15% normal guinea pig serum and 85 units of penicillin and 85 jAg of streptomycin/ml. Preparation of lymph node cell suspensions: Twelve to twenty one days after sensitization axillary, inguinal and popliteal lymph nodes were obtained aseptically from guinea pigs which had been anesthetized with ether and exsanguinated by cardiac puncture. The nodes were diced into tissue culture medium, teased gently with mouse-toothed forceps, and the resulting cell sus-pension was pipetted into centrifuge tubes. The suspension was allowed to stand for 4 min so that the tissue fragments settled by gravity. The supernatant was then removed to a fresh tube. After three such settlings the cell suspensions were essentially free of tissue fragments. The majority of cells were viable as assessed by trypan-blue exclusion, and the preparations contained 90-95% lymphocytes by Wright's stain and phase microscopy. Suspensions were adjusted to a final concentration of 1.8 X 107 cells per ml. Aliquots of these suspensions were made to contain ovalbumin, or OCBC-BGG 100 jAg/ml. Suspensions not containing antigen were also prepared. In each experiment, suspensions were incubated in specific antigen and an unrelated antigen.