A low m.w. fragment capable of mobilizing leukocytes from bone marrow is generated in serum upon activation of complement. This fragment has been reported to be derived from C3 (Rother, K., Eur. J. Immunol. 2, 550, 1972).

We have isolated a fragment (C3e) of human C3 that has a m.w. of about 10,000 daltons and consists of 101 amino acid residues, according to computer analysis of amino acid composition data. C3e was generated in two ways: by incubation of isolated C3 with 2% (w/w) trypsin for 120 min at 37°C, or by aging serum at 37°C for 5 days. C3e preparations obtained by the two methods were virtually indistinguishable with respect to molecular size, amino acid composition and antigenicity. The electrophoretic mobility of C3e at pH 8.6 is greater than that of albumin. Anti-C3e reacts with C3e, C3, C3b and C3c, but not with C3a or C3d.

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