OBJECTIVE: An Rh-negative woman with preexisting anti-D antibodies may affect some or all subsequent fetuses, depending on the genotype of her Rh-positive partner. Currently, a reliable technique for an absolute determination of RhD genotype is not available. This study was initiated to develop an accurate method for RhD genotyping in men. STUDY DESIGN:RhD genotype was determined by deoxyribonucleic acid amplification of a D-specific sequence in single sperm cell samples. Micromanipulation techniques were used for sampling of single sperm cells, which were further amplified by multiplex nested polymerase chain reaction at the RhD locus. A RhD sequence amplification product was expected in all of the successfully amplified samples from Rh-positive homozygotes, in some of the samples from heterozygotes, and in none of the samples form Rh-negative subjects. RESULTS:RhD genotype was accurately determined in 10 of 10 donors. A total of 132 single sperm cells were analyzed (8 to 17 samples per donor), of which 96 were successfully amplified as assessed by an internal control. As expected, the specific region of the RhD gene was amplified in all, some, and none of the signal-positive sperm samples from Rh-positive homozygotes, heterozygotes, and Rh-negative subjects, respectively, allowing accurate determination of the genotype. CONCLUSION: An accurate diagnosis of the RhD genotype can be attained from single sperm cell analysis by means of polymerase chain reaction and may have major clinical applications in the management of Rh isoimmunization. (AM J OBSTET GYNECOL 1996;174:1300-5.)