In the bovine lens theγIV-crystallin fraction is a principal determinant of the phase separation and opacification temperature, TC(Siezen et al, Proc. Natl. Acad. Sci. USA 82, 1985, 1701). We have now measured the effect on TCof purifiedγIV-crystallin solutions produced by a variety of reagents which affect protein-protein, protein-water and water-water interactions.Ionic strengths less than physiological increase TCdramatically, while higher ionic strength has very little effect. Calcium ion concentrations up to 8 mM produce no change in TC. Glycerol and acrylamide both depress TClinearly with reagent concentrations; TCdepression ofγIV-crystallin by these compounds is quantitatively the same as for whole lens.Sulfhydryl reducing agents such as glutathione and dithiothreitol lower TC, while hydrogen peroxide increases TC. Changes in opacification temperature ofγIV-crystallin produced by oxidizing and reducing agents are time-dependent and highly non-linear with reagent concentration.Our results clearly show that bovineγIV-crystallin is an important target protein for various reagents which are known perturbants of the opacification temperature of whole lens.The relevance of these findings to human diabetic and senile cataract formation is discussed.