A study conducted in 30 normal human colons, obtained from cadaveric kidney donors, has evidenced the presence of two polymorphic antigenic specificities, W and Z, in the secretory cells of the colon mucosa. Three phenotypes have been demonstrated so far: W-Z- (frequency: 0.17), W-Z+ (frequency: 0.23), and W+Z+ (frequency: 0.60). Specific anti-WZ and anti-W alloantibodies, independent of anti-A and B agglutinins, were found in normal sera from blood donors. No apparent correlation was found between the WZ specificities and the ABH Lewis specificities, the other blood group systems specificities, and the leucocyte group (HL-A) specificities.