The reported case of probable superfecundation is apparently the most extensively tested of all published cases: More than 40 specificities of the HLA Loci A and B, 5 of the Locus C, Bf and GLO I, furthermore ABO, MNSs, DCcCwEe, K 1--6, Fya, Fyb, Jka, Jk5, Lu3, Hp, Gc, Gm 1, 2, 4, 5, 21, Km 1, C3, Tf including C-subtypes, Pi, Bg, ACP, PGM1 including the extended polymorphism by PAGIF, AK1, ADA, 6-PGD, EsD, GPT, GALT, CAII and Hb. A reciprocal exclusion for putative father 1 and putative father 2 to the twin 2 and the twin 1, respectively, has been reached in 7 blood group systems: HLA, MNSs, Lu, Hp, Tf, Bg and GPT. The random probability for the fatherhood of an unknown third man suitable for both twins is extremely low among whites (1 : 10(8)) or blacks (1 : 10(9)). The development of paternity investigations within the last two years, mainly basing on isoelectrofocusing techniques, is shortly reported. The following systems are preferably involved: PGM1 (4 common alleles), Gc (including 1F and 1S), Pi (3 common subspecificities of M), C6, Tf (3 common subspecificities of C), FUC, Apo E, C2.