Histomonads grew rapidly in Dwyer's medium, consisting of medium 199, chick embryo extract, serum, and rice powder, reaching a population size of about 5 × 105 in 3–4 days, followed by a rapid decline. Substitution of other cell culture media (L-15, MEM, or RPMI) for M199 was also satisfactory, except for Waymouth's medium, which produced a lower and later peak of growth. Omission of serum or rice rendered media unsuitable for growth. Bacteriological culture media did not support growth of histomonads. Media that included glucose were unsuitable because the pH of the cultures dropped to about 4. The effect of glucose on pH was due to the action of bacteria. Oxygen inhibited growth of histomonads. There was no growth when culture tubes were not capped tightly, regardless of the medium used. Histomonads grew well with rice flour, cornstarch, oat flour, rye flour, and buckwheat flour. Barley and blue corn meal were less satisfactory. It appeared that the requirements for growth of the lumen phase Histomonas meleagridis included a suitable physiological saline, serum (of any source), and a starch source (grain flour). Anaerobic conditions and a pH near neutral were best. Histomonads separated into pure cultures by flow cytometry would not grow without the inclusion of an unspecified species of bacteria.