In the study of nutritional requirements of leptospirae, a basal medium containing asparagine, group B vitamins, glucose, etc. was developed. The growth of pathogenic leptospirae, Leptospira canicola Utrecht and L. icterohaemorrhagiae Mikawajima, were obtained in the basal medium supplemented with dead cells of mycobacteria, particularly of Mycobacterium smegmatis and M. phlei, in place of rabbit serum. The growths were or nearly equivalent to that obtained by the medium containing rabbit serum. When mycobacterial cells were removed by Seitz or Millipore filtration from their culture medium, leptospirae did not grow. It was believed that the growth supporting factor for leptospirae was closely associated with mycobacterial cells, and that the factor did not readily diffuse into the surrounding medium. The factor was heat-stable. The addition of rabbit serum to the medium containing, mycobacterial cells did not particularly increase the number of leptospirae cells. No other microorganisms was found to be superior to mycobacteria in growth supporting activity for leptospirae.