We report on a lifelong 'carrier' state of non-cytopathic bovine virus diarrhoea virus (BVDV) in an otherwise healthy pig. Three out of 13 pigs of a litter congenitally infected with BVDV survived for more than 3 months. One pig was BVDV seropositive at 1 month, the second seroconverted between 6 and 8 months, and the third remained viraemic and BVDV-immunotolerant until slaughter at 26 months. The latter pig, a boar, excreted virus in oropharyngeal fluid, urine and semen. Ejaculates, however, did not contain spermatozoa. The persistently infected pig had leucopenia from 3 months onwards, though differential white cell counts and thrombocyte counts remained normal. The antibody response of this pig after vaccination against foot-and-mouth disease and after infection with porcine parvovirus was not affected. The antibody response after vaccination against hog cholera, however, was delayed. In contrast, superinfection with antigenically related cytopathic BVDV strains did not alter the status of BVDV immunotolerance, nor did it induce clinical signs resembling mucosal disease as observed in cattle persistently infected with BVDV. Although the role of pigs in the epizootiology of bovine virus diarrhoea infections may be limited, the existence of healthy BVDV carrier pigs should be noted wherever control or eradication measures are applied.