To investigate the potential pathogenicity of Baylisascaris spp. nematodes, mice were experimentally infected with Baylisascaris transfuga eggs, which had been cultured in 0.1 N sulphuric acid, for a period in excess of 1 year. Infectivity for mice appeared after 2 weeks in cultures (2.8%), peaked after 4 weeks (37.4%) and then waned over the next 18 months. The implications of B. transfuga as a possible agent of visceral larva migrans in animals and humans was demonstrated.