A aerum (J.D.) obtained from a donor of type RhlRh,(CCDee.) was determined by the usual atudies to be anti-hr'(c). It was eventually found, however, to give no reaction with an occarional blood sample which was hr'-pitive with all other typing serums. Cells of the latter phenotype (J.U.negative, hr'-positive) rhow a weakened expression of both hr' and hr(f). The characteristic is inherited, is expressed in the heterozygote, and has not yet been found in the hamozygoua form. The J.D. aerum has been designated as anti-Rh26, hence the Rh phenotype identified by it is Rh: w4, w6, -26. Most hr'-positive red blood c e h also pos-IUW Rh26. One donor Jnmesaing Rh26 but not hr' has been detected.SINCE the retraction of cv.2 the Rh factor hr'(c) has not been subtyped and seldom found to be weakly expressed,3 nor has anti-hr' been considered a mixed antibody. A recently discovered serum has, however, revealed a distinct, inherited Rh phenotype, in which the expression of hr' and hr is affected, and which was initially thought to represent a variant of hr'. The SerumA potent anti-hr'(c) was obtained from J.D., a young woman with a history of having had three transfusions in 1958 for an ectopic pregnancy, followed by two stillbirths, in 1960 and 1961, apparently due to crythroblastosis. She is group 0, phenotype Rh,Rh, (CCDee); her husband is group A, phenotype Rh,rh (ccDEe). The J.D. serum had a titer of 1:16 (antiglobulin) against Rhlrh (CcDee) cells, 1 : 32 against rh (cde) cells. It was also active against saline-