Two T-strains of Mycoplasma, T-960 and T-McA, grew optimally in T-broth, pH 6.0–6.5, in screw-capped tubes. Both grew in T-broth, pH 6.0–6.5, containing ammonium salts but not at an alkaline pH. Mycoplasma T-960 and Mycoplasma T-McA had cfu consisting of four cells and single cells, respectively, that resisted sonication. Both had typical growth curves and declined rapidly when the cfu reached about 5 × 106/ml of T-broth. Aeration probably restricted the synthesis of essential metabolites and, thus, retarded growth. The concentration of cfu was increased and the decline delayed by dialysis of broth cultures during incubation. The decline was not due to lack of nutrients nor production of NH3 and changes in pH in the T-broth but to a catalase-resistant, heat-stable dialyzable toxic product. Three other laboratory strains and five wild strains also grew in T-broth, pH 6.0–6.5, containing ammonium salts. All produced and were inactivated by the toxic product.