Polyamines are small aliphatic polycations present in all cells. Polyamines are known to have several important functions, which are essential for life (Ref. 1). The cellular polyamine pool contains several modified forms of polyamines, however putrescine, spemidine and spermine are the most abundant among others. Normally polyamines exert their molecular functions by direct binding to cellular macromolecules such as DNA, RNA and proteins (Refs. 1 and 2). We have established an in vitro polyamine binding assay that enables identification and characterization of novel polyamine targets. Using this ultrafiltration-based protocol, we have assayed polyamine (spermidine or spermine) binding to yeast or human ornithine decarboxylase (ODC) antizyme (AZ) (Refs. 3 and 4) protein produced in Escherichia coli. Apart from being easy and fast, this protocol requires only small quantities of purified proteins for determination of polyamine binding. Combined with protein fractionation methods, this protocol can be applied for the identification of novel intra or extra cellular polyamine binding proteins. The protocol given below describes determination of polyamine binding for 6His-tagged S. cerevisiae ODC antizyme (OAZ1). An adopted version of this protocol was used to determine polyamine binding to human antizyme fused to maltose binding protein (Ref. 4).