9 St. Louis, Mo.; 2.2 g/L) in triple-distilled water. The pH was adjusted to 7.2 with NaOH or HC1, and the solution sterilized by passage through 0.45y detergent free Swinex-25 millipore filters (Millipore, Bedford, Mass.). For in vitro studies of primary immune responses (Mishell-Dutton type cultures [83]) a medium modified from Click et_ al_. (35) was used. Modifications of the original technique included exclusion of NaOH and 2- mercaptoethanol, substitution of RPM1 1640 for Hank's and the addition of gentamycin (20 mcg/ml), heparin (5 U/ml) and sodium bicarbonate (2.2 g/L, dissolved in the initial media preparation). Concentrations of the amino acids (GIBCO). nucleic acid precusors (GIBCO), pyruvate (GIBCO), glutamine (GIBCO), vitamins (GIBCO), penicillin and streptomycin were added as described by Click et al. (35). The medium was prepared by dissolving the above ingredients in triple-distilled water, adjusting the volume and pH and sterilizing as for the preparation of RPMI 1640 (described above). Medium Supplements * Serum and plasma sources which were tested as medium supplements for in vitro studies were fetal calf serum (GIBCO; Lot # A030113; Inter national Scientific Ind., Inc., Cary, Ill.; Lot # 7066411), Calf Serum (GIBCO; Lot # Ro266T), human serum pools (five pools furnished by Dr. R. Waldman, University of Florida, > 50 normal human sera per pool), rabbit serum pools (New Zealand White rabbits, two pools, >10 normal rabbit sera per pool), alligator (Alligator mississippensis) serum (Silver Springs Reptile Institute, Silver Springs, Fla.; four indivi dual normal alligator sera), fresh water catfish (Ictaluru cerracanthus) plasma (heparinized, pool from ten catfish), large mouth bass (Micropterus