Spermatozoa were prepared by swim up technique in the same way as for in vitro fertilization of human eggs and incubated with antihuman fibronectin antibodies in the immunofluorescence technique. Fibronectin mediates adhesive interactions between cells. 69 p.c. of spermatozoa with a normal shape and only 17 p.c. of them exhibiting a pathological shape showed a positive fibronectin-fluorescent band on the equatorial segment, called equatorial fibronectin band (EFB). The cryopreservation of semen did not influence the EFB. The significance of EFB for the sperm-oocyte-binding is discussed in connection with the sperm morphology and epididymal function.