A healthy, type MN, Caucasian woman, with no evidence of a subgroup of N or autoimmune disease, was found to have anti-N and strong anti-H in her serum. The anti-H in a sample of blood taken after delivery of her fourth child was strong enough to mask the anti-N at 22 C, whereas in pre-delivery samples it had been very weak. The case is similar to one reported by Metaxas- Biihler and associates2 in 1961, and another by Greenwalt and colleagues1 in 1966. A family study is presented, and the MN antigen is investigated in detail. The reason for the phenomenon remains unexplained.