Membrane and cytoplasmic fractions of Mycoplasma hominis inhibited the multiplication of this mycoplasma. Arginine deiminase (EC 3.5.3.6), isolated from both fractions, reproduced the inhibition. The purified cytoplasmic deiminase had a subunit Mr of 49,000, a specific activity of 53 units (mg protein)-1 and an A280/A260 ratio of 1.76. The membrane-associated enzyme had an identical Mr but lower values for specific activity [39 units (mg protein)-1] and the A280/A260 ratio (1.46). In experiments in vitro, recent clinical isolates of M. hominis produced less arginine deiminase, but grew faster than the laboratory reference strain PG 21. In addition, other growth inhibitory components associated with membrane preparations were detected in recent clinical isolates but were absent from strain PG 21.