Aqueous solutions of a series of aminopteridines (5—10 mg./ml.) stable at physiologic pH's were prepared either by salt formation with hydrochloric acid or by complex formation with 10 percent deoxyribonucleic acid. Deoxyribonucleic acid was superior to ribonucleic acid, crystalline bovine serum albumin, human serum albumin fraction V, or gum acacia in solubilizing 4,7-diamino-2( p -chlorophenyl)- N -(2-diethylaminoethyl)-6-pteridinecarboxamide at pH 7. Potentiometric titration detected complex formation between deoxyribonucleic acid and the above aminopteridine.