In vitro fertilization and embryo transfer were performed for rederivation of four strains of mice harbouring mouse hepatitis virus (MHV) and/or Pasteurella pneumotropica (P. pneumotropica). Superovulated oocytes were fertilized by preincubated cauda epididymis sperm in vitro. Fertilized eggs at 2-cell stage were transferred into the oviducts of specific pathogen free (SPF) recipients. Microbial examination of sperm and/or oocyte donors verified the presence of P. pneumotropica and/or of antibodies to MHV in all strains, but neither in the recipients nor in the offspring antibodies to MHV could they be detected. The results indicate that an in vitro fertilization-embryo transfer (IVF-ET) system is an effective and simple alternative to cesarean operation in infected mice.