Influenza-B virus was identified in 102 children admitted to hospital during two epidemics in 1973 and 1974, enzbling the symptomatology of infection with this virus to be assessed in detail for the first time. Abdominal pain, often severe enough to require differentiation from acute appendicitis, emerged as a dominant symptom, especially in older children. Respiratory symptoms were often insignificant, although the lower respiratory tract was sometimes involved. Other symptoms in some children included convulsions and acute myalgia. The immunofluorescent method of virus diagnosis was found to be reliable for influenza B, except in a few cases ehere nasopharyngeal secretions were scanty, giving 97-5 percent copositivity with standard isolation techniques. The rapid result provided by immunofluorescence was helpful in clinical diagnosis and management and also in the control of hospital cross-infection.