Purified preparations of skin collagen, tendon gelatin, and thymus histones, isolated from various animals administered 75Se-labeled selenite, contained only a trace of cystine (plus [Formula: see text]-cystine) and incorporated very little 75Se activity. Keratins and milk proteins, which contained relatively high levels of cystine, also had high radioselenium levels. The data indicate that the cystine content of a protein may be a factor which affects selenium incorporation, possibly by the formation of selenotrisulfides (P–S–Se–S–P).