An experimental teleradium unit is described, in which it is arranged that focal “skin” distance, thickness of filter and area of source, may be varied. It is shown experimentally that the amount of soft radiation at the aperture increases with focal “skin” distance, as does the percentage depth dose. Results of experiments with varying thicknesses of lead filter show that the first 0·5 mm. of lead, together with 0·6 mm. brass, produce the maximum improvement in depth dose. Varying the area of the source produces little difference in depth dose for large focal “skin” distances, but for smaller ones (2–3 cm.) increases the depth dose by some 20 per cent.It appears that the inverse square law alone does not predict accurately the depth dose observed, and that an apparent absorption coefficient equal to 0·25 cm.−1is also required to bring the theory and experiment into agreement for heavily filtered sources.