Antibodies to rabbit spermatozoa isolated from immunized virgin female rabbits were conjugated to hemocyanin and have been used to determine the pattern of antigenic sites over the surface of rabbit spermatozoa under a variety of conditions. When the spermatozoa are incubated with antibody at 0 degrees C, labeling is almost entirely restricted to the postacrosomal sheath. Incubation at room temperature (22 degrees C) or at 37 degrees C results in progressively increased acrosomal labeling as well as enhanced postacrosomal sheath labeling. Initial incubation at low temperature followed by washing and high temperature incubation yields cells which indicate little tendency for postacrosomal sheath sites to migrate toward the acrosomal region. Absorption of the antibody preparation with rat lymphocytes has no effect on labeling density, whereas absorption with rabbit lymphocytes diminishes the intensity of labeling and yields cells with markers primarily over the postacrosomal sheath. It is concluded that the postacrosomal sheath plasma membrane has specific antigenic properties distinct from other regions of the head.

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