Antisera produced against Medicago radiata, Medicago lupulina, Melilotus officinalis, and Melilotus indica were reacted against these species after preabsorption with representative species of Medicago, Melilotus, and Trigonella. The serological data from immunodiffusion and immunoelectrophoretic tests confirm a close serological relationship among taxa of the three genera. The results support and emphasize the isolated positions of M. lupulina, Medicago secundiflora, and M. radiata vis-a-vis the remaining species of Medicago. The first two species show serological affinities to both Medicago and Melilotus and appear to be in an intermediate position relative to these two genera. Medicago radiata is serologically more distinct from other Medicago species than from Trigonella taxa.