123I for use in clinical diagnostic procedures has been produced by bombarding antimony with 40 MeV 3He particles. The purity of 123I was measured with a Ge(Li)spectrometer and a low energy Ge(Li) detector. The 123I-product was followed for 1.5 months with a well-type NaI scintillation counter to establish the half-life and confirm the identity of the nuclides. In order to minimize the yield of 121I the optimum cooling time and chemical procedures are presented. After 20 h cooling, the 121I, 124I and 125I contaminants were about 1.8, 4.3 and 0.04% respectively. These values were in agreement with those calculated from the thick-target yield curves. The calculated absorbed dose to the thyroid by the 123I and by-product nuclides were only 4.3 and 7.3% of 131I. 123I could be measured at the window of 99mTc of a NaI scintillation camera and a curie-meter with an error of 10%.