Channel-forming integral protein (CHIP) is the archetypal member of the Aquaporin family of water channels. Delayed CHIP expression was shown recently in perinatal rat (Smith, B. L., R. Baumgarten, S. Nielsen, D. Raben, M. L. Zeidel, and P. Agre. 1993. J. Clin. Invest. 92:2035-2041); here we delineate the human patterns. Compared with adult, second and third trimester human fetal red cells had lower CHIP/spectrin ratios (0.72 +/- 0.12, 0.94 +/- 0.22 vs 1.18 +/- 0.11) and reduced osmotic water permeability (0.029, 0.026 vs 0.037 cm/s); CHIP was already present in human renal tubules by the second trimester. A patient with a novel form of congenital dyserythropoietic anemia (CDA) with persistent embryonic and fetal globins and absent red cell CD44 protein was studied because of reduced CHIP-associated Colton antigens. Novel CDA red cells contained < 10% of the normal level of CHIP and had remarkably low osmotic water permeability (< 0.01 cm/s), but no mutation was identified in Aquaporin-1, the gene encoding CHIP. These studies demonstrate: (a) unlike rat, human CHIP expression occurs early in fetal development; (b) red cell water channels are greatly reduced in a rare phenotype; and (c) disrupted expression of red cell CHIP and CD44 suggests an approach to the molecular defect in a novel form of CDA.