The preparation of a semisynthetic tritiated insulin with a specific radioactivity of up to 20 Curies per millimole.

1. This paper describes the semisynthesis of [3H]insulin of high specific radioactivity which is indistinguishable from native insulin in its hypoglycaemic activity towards mice. 2. The maximum specific radioactivity so far obtained is 20 Ci/mmol. At this value the recommended scale of preparation is between 1 mCi (30 mug of product) and 10mCi (300 mug of product). Reaction volumes are then of the order of 10 mul. 3. Special problems arise when the techniques of peptide synthesis are applied to the production of such small quantities of material. We describe these problems and discuss the means of overcoming them.