The precipitin reaction, as measured in liquid medium (Boyden method) and double-diffusion gel method (Ouchterlony), was used as an index for estimating degree of serological correspondence in selected taxa of the Magnoliaceae. Proteins were saline extracted from seeds of 13 species and one variety of Magnolia, from four populations of M. virginiana, and from one species each of Manglietia, Michelia, Talauma, and Liriodendron. Serologically, Magnolia, Manglietia, Michelia, and Talauma can be grouped together, with the greatest serological similarity being between Magnolia and Michelia. Liriodendron was quite distinct from the other genera, although all have one antigen-antibody component in common. Although interspecific tests in Magnolia placed M. obovata and M. tripetala very close together, they were distinguishable by some of the precipitin tests. There was very little serological correspondence between M. portoricensis and the other 12 species of Magnolia tested; in fact, the correspondence was less ...