Inertial migration has been used to fractionate feed streams containing latex particles and sheep leukocytes. For latex particles, a purified stream of small particles, less than 4 mum, was obtained. For sheep leukocytes, a stream containing almost no large leukocytes greater than 16 mum was produced. The results obtained show that this method could be used to fractionate biological cells of different sizes. Inertial migration is most suitable for the removal of larger particles which contaminate a dilute suspension of smaller particles.