Sperm-specific nuclear protamines are dissociated before decondensation of sperm nuclei during fertilization in pigs. In the present study, replacement of nuclear protein by histone in boar spermatozoa during in vitro fertilization was evaluated by immunohistochemistry using anti-histone antibody. First, the specificity of the antibody used in this study was examined. Immunohistochemistry of the testes and epididymides indicated that somatic nuclei, but not elongated spermatids or maturing spermatozoa, were immunoreactive. Furthermore, immunoreaction was diminished after the antibody had been preincubated with unfractionated histone, indicating that the antibody was specific for the somatic nuclear histone. Immunohistochemistry of serial sections of oocytes, which were matured and co-cultured with boar spermatozoa for 2 to 6 h indicated that, at 2 to 3 h after insemination, penetrating sperm nuclei in the condensed state were not immunoreactive. At 4 to 5 h after insemination, some of the condensed sperm nuclei were immunoreactive in part or over the whole area of the nucleus, and all of the decondensing nuclei and male pronuclei were immunoreactive. At 6 h after insemination, the decondensing sperm nuclei and well-developed male pronuclei were immunoreactive. These results imply that, in pigs, remodelling of sperm nuclear protein from protamine to histone is initiated at the time of sperm penetration, before onset of decondensation and male pronuclear formation.