Phenylarsenious acid and diethyltin, which are known to have a great affinity for dithiol groups (Aldridge & Cremer, 1955; Peters, 1946), would be expected to compete with triethyltin for the two cysteine components of the triethyltin-binding sites. This was observed, although the affinities obtained with phenylarsenious acid and diethyltin for untreated cat haemoglobin (2x 1 0 4 ~ ' and approx...