Abstract
Established techniques with broad bean roots were used to measure the relative biological efficiency (R.B.E.) of Co60 gamma rays from a kilocurie therapy unit, and X rays generated at 220 kVp. The value of the R.B.E. was 0.84 with 95 % confidence limits of [plus or minus]0-05. There was no significant difference in the R.B.E. when the irradiation with the Co60 gamma rays was carried out 1.5 cm or 20 cm deep in a water-filled phantom.