The effect of lithium chloride on the circular dichroism spectra of α-chymotrypsin, δ-chymotrypsin, di-isopropylphosphoryl(DIP)-α-chymotrypsin, and chymotrypsinogen at pH 4.0 has been investigated. The spectra showed well-resolved bands at 229, 262, 287, 291, 296, and 305 nm. The 229 band becomes less negative with increasing ionic strength in α- and δ-chymotrypsins, is unaffected in DIP-α-chymotrypsin and chymotrypsinogen, and is probably associated with the changes in reactivity of the N-terminal isoleucine-16. The bands at 262 and 287 nm are affected by lithium chloride in α-chymotrypsin and in DIP-α-chymotrypsin. These changes appear to be associated with dimerization. The 291 band becomes more positive with increasing ionic strength in all three forms of chymotrypsin, but not in the zymogen. The 296 nm band has the same ellipticity in all four proteins and is unaffected by changes in ionic strength. The same applies to the 305 nm band. Evidence is presented which suggests that the changes in the 287 nm transition are due to changes in the environment of tryptophan-215.