We have used an electrophoretic retardation assay to investigate the interactions of wheat high mobility group (HMG) proteins with DNA and with isolated trimmed mononucleosomes (complexes which contain a histone octamer and approximately 146 base pairs of DNA). In order to characterize these interactions, we have compared the binding of each of the wheat HMG proteins, HMGa, b, c, and d, with those of the low molecular weight chicken HMG proteins HMG14 and 17. These vertebrate animal HMG proteins have previously been shown to occupy two specific binding sites on animal nucleosomes and to have a greater affinity for nucleosomes than for naked DNA (Mardian, J. K. W., Paton, A. E., Bunick, G. J., and Olins, D. E. (1980) Science 209, 1534-1536; Sandeen, G., Wood, W. I., and Felsenfeld, G. (1980) Nucleic Acids Res. 8, 3757-3778). As a criterion for "specific binding," we have used the property of HMG14 and 17 binding of causing a discontinuous shift of nucleosomes to a distinct band of lower electrophoretic mobility. According to this criterion, wheat HMGb, c, and d do not bind nucleosomes specifically. These HMG proteins have approximately the same affinity for nucleosomes and naked DNA. Wheat HMGa does bind nucleosomes specifically by this criterion, but other aspects of the binding are reminiscent of histone H1-nucleosome binding. We present evidence that trimmed mononucleosomes of wheat are conformationally distinct from their animal counterparts. Despite the conformational differences, competition studies indicate that chicken and wheat mononucleosomes have essentially identical affinity for the low molecular weight animal HMG proteins.