Renin substrate (angiotensinogen) in unfractionated human plasma has been shown to exist in multiple forms by DEAE-cellulose chromatography and isoelectric focusing. Two major and 5 to 6 minor peaks were resolved by using a descending pH gradient elution from DEAE-cellulose columns. The two predominant forms were eluted at or near pH 4.8 and 4.4 and usually accounted for 40--50% of the recovered substrate activity. The elution pH values of the various forms were nearly constant among plasmas from normal males and females and in diabetes, early and late pregnancy and estrogen substitution therapy. The relative distribution of components was not affected by prior freezing of the plasma or by dialysis against the column buffer. Eight renin substrate forms were clearly resolved during isoelectric focusing of plasma from a woman on estrogen substitution therapy. Four of these focused at pH 4.79, 4.88, 4.94 and 5.02 in 1% (w/v) ampholytes pH 3.5--5 and were nearly equal in substrate amount. Together these 4 forms constituted 66% of the total recovered activity. A similar pattern but with decreased amounts of each form of substrate was obtained with normal plasma.