Abstract: In radiotherapy use is made of apparatus of the type in which a chain of balls comprising radioactive balls and inert spacer balls is moved pneumatically to and from an applicator tube within the patient, the spacer balls are made of a material such as nylon which is softer and less dense than the radioactive balls, and is radiographically transparent. This reduces wear and tear on the radioactive balls and facilitates control of dosage and monitoring of the treatment. The balls are sorted after use by means of a radiation detector. The ball chain has a large diameter end ball of ferromagnetic material, which blocks air flow through the applicator when correctly located therein, and whose position can be verified by an induction coil.