A reliable method for radioiodinating antibodies is described. The reaction is initiated by the addition of a fine suspension in buffer of iodogen particles formed in a novel way. The addition of an acetone solution of Iodogen to phosphate-buffered saline yields a uniform suspension of 3.0 micron diameter particles. This preparation has been used to label polyclonal anti-prostatic acid phosphatase (PAP) antibodies with up to 185 MBq of iodine-123 mg-1. Labelling efficiencies of 92% are achieved in a reaction time of less than 5 min. Such labelled antibodies are expected to be of use in the immunoscintigraphy of patients with prostatic cancer. The reaction parameters have been optimized and the method is particularly suitable for routine use.