SRS-A (Slow Reacting Substance of Anaphylaxis) is one of the mediators involved in bronchial asthma. The studies of other investigators and our own (ZO1 HL 1946-02 HE, 1975) have provided evidence to suggest that SRS-A contains sulfur. The purpose of the present study is to determine whether SRS-A contains thiol or disulfide groups.