Transgenic mice expressing human HbC have a higher volume-stimulated K-Cl cotransport activity than HbA or C57BL controls. We determined the volume-stimulated K-Cl cotransport activity in red cells from HbCKO, HbC-L, HbAKO, and C57BL mice by calculating the difference between the flux in 330 mOsm isotonic Cl^- versus 250 mOsm hypotonic Cl- media, which yields the volume-stimulated K⁺ efflux or K-Cl cotransport activity as described in “Materials and methods.” The figure shows the average ± SE of 3 different experiments performed on at least 4 different mice per mouse type (HbC versus HbA or C57, P < .02).