As a result of screening procedures employed for animals entering the AI service, two bulls were identified as being persistently infected with bovine virus diarrhoea virus by isolation of the virus from blood. Semen was collected on two occasions from these bulls; its quality as measured by density and motility was poor. Gross abnormalities of the sperm head, termed 'collapsed' heads, were seen in 28 to 45 per cent of sperm from one bull and in 1 per cent of sperm from the other. The collapsed heads were small and the whole head or its anterior part had the appearance of a dried pea. Electron microscopy showed the defect to consist of convoluted nuclear material with membrane-bound vacuoles and invaginations containing membranous debris and lamellar structures. In the 'high incidence' bull there was a corresponding increase in enlarged sperm heads. The 'low incidence' bull had sperm with heads of similar mean size to sperm from control bulls but with an increased variance. The semen was diluted in a lactose diluent, frozen and stored in liquid nitrogen. The distribution of viral antigen was determined and virus was isolated from several fractions of the semen, both before and after processing and cryopreservation. In one animal raw semen failed to yield virus but virus was recovered after processing, suggesting that raw semen may not be suitable for the efficient detection of the virus.