T-mycoplasma strains no. 960 and T-McA grew optimally on T-agar, pH 6.0–6.5, containing 10% horse serum when incubated in an atmosphere containing ⩾ 5% CO2, Growth was retarded by aerobic incubation but not by anaerobic or microaerobic incubation. Both size and number of colonies were affected by the volume of T-agar in the petri dish. Colony size only was below normal on T-agar, pH 6.0–6.5, containing ammonium salts, 5% and 20% horse serum, and especially at an alkaline pH, but it was unaffected by addition of hepes buffer (N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid) or an extra 6 mg of urea per 100 ml. Colonies of strain no. 960, T-McA, three other laboratory and five wild strains could be changed from rough colonies averaging 43 µm in diameter to “friedegg” or smooth colonies 100 µm in diameter. These variations were dependent on the T-strain, type and concentration of agar and of MgS04, and substances in Ionagar. These findings suggest further that T-mycoplasmas have different requirements for growth, and strain-to-strain differences are evident.