Using a modification of the Barrnett and Seligman histochemical procedure, the proportion of protein-bound S—S/SH Was measured in the chromatin of onion root tip cells at three different stages of mitosis as well as in the nucleolus. Inconsistencies between experiments led to studies of the nature of the histochemical reaction before and after sections were reduced in thioglycolate. Different fixatives and ribonuclease were tested for their effects on the outcome of the reaction.