Calf thymus DNA, nucleic acids extracted from bacterial cells and animal tissues, amino acids, and porcine gastric mucin were tested as growth factors for mycoplasmas. The mycoplasma strains used were M. pneumoniae CL. FH, M. salivarium-C Hup 127, and M. orale N-1. These strains were kindly supplied by Dr. Hayflick.The results obtained are as follows.(1) DNA and extracted nucleic acids stimulated the growth of the mycoplasma strains, especially M. orale N-1. The growth of the strain was strongly influenced by the addition of nucleic acids extracted from animal tissues.(2) Of the amino acids tested, l-glutamine, l-lysine, l-methionine, l-phenylalanine, l-threonine, and l-arginine enhanced the growth of mycoplasmas. These enhancing effects, however, should be further studied individually, because the PPLO broth tested was found to contain hitherto unknown amino acids.(3) Porcine gastric mucin had a stimulating effect on the growth of M. salivarium and M. orale, but no effect on M. pneumoniae. The stimulating effect of the mucin was found in the fraction of acid mucopolysaccharide obtained from fractionation by means of diffusion in a Pevikon C-870 column.