Presence of a Tetrahymena growth promoting activity in fetal bovine serum

Treatment of cultures of the ciliate Tetrahymena with fetal bovine serum (FBS) enhanced the rate of cell proliferation. The growth promoting activity was partially purified from FBS as a high Mr complex including four components with apparent Mr values of 180 kDa, 68 kDa, 60 kDa and 30 kDa by a 4‐step procedure. The 180 kDa component was identified by amino acid sequencing as α2‐macroglobulin. The addition of purified α2‐macroglobulin from bovine plasma to cultures of Tetrahymena was also found to enhance the rate of cell proliferation. This report is the first dealing with the direct identification of a mammalian factor which promotes the growth of free‐living protozoa.