Fractionation of several standard preparations of human growth hormone (hGH) by polyacrylamide gel electrophoresis (PAGE) reveals a multiplicity of bands identified by relative mobility (Rf) values. Four major components (B, C, D and E) were physically characterized at pH 7.8, 8.8, 10.2 (0 C) and 9.5 (25 C): They are indistinguishable in molecular size, but show a progressive increase in molecular net charge from B to E. Species B predominates in hGH preparations of relatively low prolactin bioactivity, whereas species C, D and E predominate in those of high prolactin activity. (Endocrinology93: 848, 1973)