Although many infants with intraventricular haemorrhage (IVH) recover without hydrocephalus, little is known about how blood is cleared from the CSF pathways. Fibrinolytic activity was measured by the fibrin plate method in CSF from 11 normal infants and 17 infants with IVH. Normal CSF showed no fibrinolytic activity. All the samples taken less than 17 days from diagnosis of IVH failed to show fibrinolytic activity. All but 1 of the CSF samples taken between 17 and 60 days of IVH showed fibrinolytic activity. In 1 infant where 14 serial samples were taken, there was no detectable fibrinolysis up to 16 days after IVH but from 19 to 52 days there was definite fibrinolytic activity. Delayed endogenous fibrinolysis in the CSF is common after IVH but may, in some cases, be insufficient to prevent hydrocephalus.