Within the richmondenine-emberizine-thraupid assemblage, _Junco_ and _Zonotrichia_ constitute a sub-group apart from the others. _Piranga_ and _Richmondena_ show close serological correspondence. The present taxonomic position of _Spiza_ in the Richmondeninae, which has been questioned by Beecher (1951a:431; 1953:309), is corroborated at least insofar as the serological evidence is concerned. Certainly, serological correspondence of _Spiza_ with the richmondenine-emberizine-thraupid assemblage is greater than with any other group of birds tested. It is obvious that the serological affinities of the carduelines do not lie with the richmondenines, emberizines, or thraupids. The carduelines show greater serological correspondence with the estrildines than they do with any of the other groups tested. Further serological investigation involving other species, however, is necessary before the nearest relatives of the carduelines can be determined with certainty. The two estrildines tested (_Estrilda_ and _Poephila_) show close serological relationship. Their nearest relatives, serologically, seem to be the carduelines. The classification (Wetmore, 1951) that places _Passer_ in the same family with the estrildines is not upheld by the serological data available. _Passer_ is not, serologically, closely related to any of the birds tested. It is of interest that Beecher (1953:303-305), on the basis of jaw musculature, places _Passer_ and the estrildines in separate families (Ploceidae and Estrildidae, respectively). _Molothrus_ shows greater serological correspondence to the richmondenine-emberizine-thraupid assemblage than to any of the other birds tested. It is definitely set apart from this group, however, and its position, serologically, is compatible with that based on evidence from other sources. There seems to be but little argument among ornithologists that icterids, fringillids, and ploceids constitute families which are distinct from one another. If, then, the serological differences between _Molothrus_ (Icteridae) and _Richmondena_ (Fringillidae), between _Molothrus_ and _Zonotrichia_ (Fringillidae), and between _Richmondena_ and _Poephila_ (Ploceidae) are indicative of family differences, there are four families represented by the birds involved. _Molothrus_ represents one family; _Piranga_, _Richmondena_, _Spiza_, _Junco_, and _Zonotrichia_, a second; _Estrilda_, _Poephila_, _Carpodacus_, and _Spinus_, a third; and _Passer_, a fourth. CONCLUSIONS The heterogeneity of the Family Fringillidae has been emphasized by many authors. The relationships of the species now included in this Family have been the subject of much discussion and constitute an important problem in avian systematics.