A growth promoting factor is contained in the petroleum ether or acetone extracted residue of lyophilized dry egg yolk. Egg white, horse serum, soybean powder, bovine serum albumin, egg albumin and milk were used in M. lepraemurium culture attempts as protein sources instead of yolk lipoprotein. None of these substances promoted the growth of M. lepraemurium. One percent egg white medium was prepared from the mixture of one part 1% Ogawa basal medium to two parts egg white, adjusted to pH 6.1. This medium does not permit the growth of M. lepraemurium but permits bacillary survival for two months. This medium is most suitable as a minimal medium to investigate growth factors of M. lepraemurium. Utilizing the minimal medium, the following substances were tested for growth promoting activity: lecithin, cholesterol, petroleum ether extracted fraction of yolk, butanol extracted fraction of yolk, retinol, hemin, yeast extract, broth, farnesol and dolichol fraction of chicken liver. None of these supported growth of M. lepraemurium. The following neutralizing agents of free radicals were tried in the minimal medium: triethylenediamine, beta-carotin, potassium iodide, potassium bromide, 2-aminoethylisothio-uranium-bromide, 2-aminoethyl-isothio-uranium-bromide-hydrobromide and cysteamine. None of these supported growth of M. lepraemurium.