Sera containing sperm-agglutinating and/or complement-dependent sperm-immobilizing activity were tested for their ability to induce a shaking phenomenon in the sperm-cervical mucus contact (SCMC) test. Donor spermatozoa were treated with the sera in a one-step incubation and washing procedure. The percentage of motile pretreated spermatozoa showing the shaking phenomenon (S%) was determined in the SCMC test. In addition, the immunoglobulin class of antibodies present on the pretreated spermatozoa was determined in mixed antiglobulin reaction tests for IgG, IgM, and IgA. An S% of at least 80 was always observed with spermatozoa pretreated in sera with a sperm agglutination titer of at least 32, provided that IgG was detected on more than 90% of the motile spermatozoa. The high S% was also obtained with spermatozoa pretreated in purified IgG from sera containing antispermatozoal antibody activity. It was concluded that the reduced cervical mucus penetration capacity of spermatozoa pretreated with sera containing antispermatozoal IgG could be ascribed, at least partially, to the occurrence of a shaking phenomenon.