NZB and B/W hybrid mice develop compensated hemolytic anemia during the first year of their life. By the age of 3-5 months, their erythrocytes show evidence of spherocytosis, increased osmotic fragility and decreased whole cell deformability, as measured by ektacytometry, a laser diffraction technique. The presence of spherocytes with decreased surface area/volume ratio was confirmed by scanning electron microscopy and osmotic gradient ektacytometry. Whereas these abnormalities persisted and worsened in the NZB mice with further growth, they gradually improved and reverted to normal by the age of 12 months in B/W mice. This spontaneous improvement seems to be due to the accumulation of red cell membrane lipids reflecting the lipemia of immune complex nephritis in B/W mice. The implications of these findings in the modulation of autoimmune hemolytic anemia are discussed.