Cholesterol requirement for growth of mycoplasmas was tested in a serum-free medium supplemented with albumin, L-arginine, palmitic acid, and various concentrations of cholesterol dissolved in Tween 80. In cases in which Tween 80 was shown to inhibit growth, the test medium was supplemented with cholesterol dissolved in ethanol. Of the 31 species examined, all but Mycoplasma laidlawii, M. granularum, and Mycoplasma species strain S-743 exhibited a growth response to cholesterol. No requirement for cholesterol could be shown with the stable L-phase variants of Streptobacillus moniliformis and Proteus species. The results provide experimental support for the view that the large majority of the established Mycoplasma species require cholesterol for growth. strain designation Mycoplasma salivarium PG-20 M. fermentans PG-18 M. pneumoniae FH M. orale 1 CH 19299 M. orale 2 CH 20247 M. orale 3 DC 333 M. lipophilum MaBy Mycoplasma sp. Navel M. neurolyticum type A M. pulmonis PG-34 M. arthritidis PG-6 M. bovigenitalium PG-11 M. bovirhinis PG-43 M. hyorhinis BTS-7 M. mycoides var. capri PG-3 M. canis PG-14 M. maculosum PG-15 M. edwardii PG-24 M. felis Cat 27 M. gallisepticum S6 M. gallinarum PG-16 M. iners 640 M. meleagridis 886 M. anatis 1340 M. laidlawii PG-8 M. granularum BTS-39 Mycoplasma sp. S-743