A hypoalbuminemic substance was extracted from mouse Ehrlich solid carcinoma. The tumors were homogenized with saline solution, centrifuged, and the resultant turbid supernatant (F-1) was separated. The turbidity was removed by freezing and thawing of the supernatant, and the resultant clear extract (F-2) was dialyzed against 12mM NaCl solution. The inner solution possessing hypoalbuminemic activity was freezedried. This fraction (F-3) was gel-filtered through Sephadex G-150, by using 12mM NaCl solution followed by 1M acetic acid containing 12 mM NaCl as the eluant. An active fraction (F-4-II) was gel-filtered again under the same condition. Of four peaks obtained the third was the active fraction (F-5-III). The yield in protein was 3.0% for F-1 and 0.2% for F-5-III. The decrease in serum albumin was statistically significant for F-1 and F-5-III, while the changes in serum globulins were not significant. F-5-III did not produce a significant change in total serum protein. F-5-III was not electrophoretically homogeneous, although it sedimented as a single peak in ultracentrifugal analysis. Its molecular weight was roughly estimated to be 16000 by ultracentrifugation. Amino acid content of F-5-III was 59.95%, and asparticacid, glutamic acid, and isoleucine were contained in fairly large amounts. Neither the extract prepared from musculus glutaeus maximus of a normal mouse by the same procedures nor bovine serum albumin displayed hypoalbuminemic activity.