A total of 11 techniques including 57 items for zygosity diagnosis was employed in two sets of triplet. Namely, placenta-fetal membrane, LDH isoenzymes, etc. The authors made an investigation in detail on the pedigree history of polyembryony and systematic analysis of two sets of male triplets. The conclusion was that the first triplet came from a single ovum, whereas the second triplet developed from double ova. Different methods for the zygosity diagnosis were discussed. It was considered that there was hereditary influence in the occurrence of triplets developing either from a single ovum or from multiple ova.