Between May and August 1975, rotaviruses were detected in the stools of 76 out of 174 (44%) newborn babies in the maternity unit at this hospital. Infection occurred less frequently in breast-fed than in bottle-fed babies (P less than 0.001). However, only 7 out of 76 (8%) babies who excreted rotaviruses had symptoms and these were mild. Complement fixation tests did not show any apparent difference in the antibody titres or serological responses between mothers of rotavirus positive or negative babies. When 68 faecal extracts known to contain rotaviruses by electron microscopy were inoculated by centrifugation on to monolayers of continuous pig kidney cell cultures (IB-RS-2), rotavirus antigen was detected by immunofluorescence in 65 (95.5%) specimens, 58 being positive after centrifugation at 3000 g and a further 7 after centrifugation at 10 000 g. Antigen was first detected 6 hours after inoculation of specimens, maximum levels being detected at 24 hours.