Rabbit antiserum to the mouse major urinary protein identified a single antigen that was also found in mouse serum and pelt extract. The skin test reactivity of mouse-pelt extract and mouse urine in two mouse-allergic subjects was significantly reduced after immunoabsorption with the gamma globulin fraction of this antiserum. The antigen defined by this antiserum was designated mouse allergen 1 (MA1). An immunoelectrophoretic procedure was set up to measure its concentration. MA1 had a molecular weight of approximately 19,000 on Sephadex gel filtration and 18,000 to 21,000 on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Isoelectric focusing identified at least four bands with antigenic activity; the major band had an isoelectric point of 3.9. Significant antigenic and allergenic activity of MA1 was retained on reduction and digestion with papain and pepsin. Heating at 90 degrees C for periods up to 180 minutes resulted in a progressive loss, but not abolition, of activity. Serum and urine derived from male mice contained approximately fourfold more MA1 than samples derived from female mice. Urine contained at least 100-fold more MA1 than serum. Of the tissue extracts studied, liver extract had the highest amount of MA1. The immunochemical properties of MA1, its tissue distribution, and sex differences in its concentration provide strong evidence that MA1 is identical to the previously described mouse major urinary protein.