Summary Neuraminidase activity could be demonstrated in highly concentrated preparations of human parainfluenza 1 virus, strain C 35 (HA2 virus). Among the substrates used, the most suitable are N-acetyl neuramin lactose and fetuin. Mucin type I and type II were not hydrolysed. The neuraminidase exhibited some characteristics similar to those of the other paramyxoviruses (Sendai, NDV, mumps, human parainfluenza 2 virus): the optimum pH ranged between 5 and 5.4, and the K m value was 5 × 10-3 m when tested with N-acetyl neuramin lactose. Its optimum activity was between 37 and 40 °C and it was thermolabile, the enzymic activity being reduced to 50% in 5 min at 45 °C and entirely destroyed at 50 °C in the same period. The thermal inactivation constants of neuraminidase and haemagglutinin and the temperature which inactivated 50% of both these activities were very similar to those already shown for NDV. Haemagglutinin and neuraminidase activities were rapidly destroyed by ionic detergents, but not by non-ionic detergents.