The concentrations of spermatozoa, fructose, IgG, IgA, albumin lactoferrin, transferrin, secretory piece of IgA, Beta 1C/Beta 1A-globulin (C'3-component of complement), ceruloplasmin and fibrinogen were evaluated in human split ejaculates and/or in whole human seminal plasma. The concentrations of spermatozoa, IgG, IgA, albumin and transferrin decreased from the first portion of the split ejaculate to the last, indicating that these proteins originate mostly from secretions other than the seminal vesicles. By contrast, the highest amounts of fructose and lactoferrin were present in the final portion of the split ejaculates, showing their seminal vesicle origin. No secretory piece, IgM, Beta 1C/Beta 1A-globulin, ceruloplasmin of fibrinogen could be detected in human semen. An unidentified antigen was found that has a relatively high molecular weight and shows Beta 1-mobility on immunoelectrophoresis.