SUMMARY: Mycoplasma laidlawii strain A did not grow significantly and sometimes died in a peptone + yeast extract (PY) medium; good growth was obtained when 1% serum or 1% bovine plasma albumin was added. The addition of cholesterol and some other steroids (at 20 μg./ml.) to the basal PY medium gave some growth although less than that given by adding serum or albumin. Certain other steroids (e.g. Δ1,4 and Δ1,4,6-cholesten-3-one) were potent inhibitors of cholesterol-promoted growth at one-tenth the cholesterol concentration. One of these steroids at 2 μg./ml. inhibited the growth which was obtained in PY + albumin medium; this inhibition was annulled by 200 μg. cholesterol/ml. Two of the inhibitory steroids at c. 300 μg./ml. inhibited the growth of M. laidlawii strains A and B, M. mycoides var. capri and M. bovigenitalium in a rich serum-containing medium. The inhibitory effects of one of these steroids was greatly diminished when it was added after growth had begun.