A formula is presented for approximating the per cent depth distribution resulting from any portal size60Co beam at 80 cm SSD. This equation may be used to obtain computer print-outs of combined multifield dose distributions as described previously (Sterlinget al., 1963b), or, in a new programme, can be used to print out the combined isodose curves directly to scale with small error.A distinct geometric property of the per cent depth dose distribution is discovered and described without attempting to explain it.