Between May 1, 1976, and May 14, 1977, 343 (32.5%) of 1056 5-day-old babies in newborn nurseries excreted rotaviruses. The infection-rate was highest during winter (49%). 76% of infected babies at this time were bottle-fed. 41% of neonates excreted low amounts of virus (less than or equal to 10(8) particles/g faeces); older children tended to excrete greater than 10(10) particles/g faeces. Infected breast-fed babies excreted less virus than those who were bottle-fed. Stools of breast-fed babies often contained clumps of complete "smooth" rotavirus particles. When the newborn nurseries were transferred to a newly built hospital wing, infection appeared in the new wards, including those admitting only new patients, within a short period. Infection was either mild (8%) or symptomless (92%), and even babies with symptoms required no treatment.