An alanine, lysine and glutamic acid-rich nuclear protein (P2) of Mr approximately 19,500 co-extracts with the histones from nuclei of Physarum polycephalum when using the CaCl2 method for histone extraction  and was found to have the composition previously ascribed to a putative histone H1(0) isolated from microplasmodia using 5% PCA (Yasuda, H., Mueller, R.D., Logan, K.A. and Bradbury, E.M. (1986) J. Biol. Chem. 261, 2349-2354). P2 has very similar electrophoretic properties to chicken erythrocyte histone H5, calf thymus histone H1(0) and the Physarum HMG-like protein AS-2, but does not appear to be immunologically or structurally similar to H5 or H1(0). An increase in the abundance of P2 was observed during exponential growth in microplasmodia, reaching an approximately 1:1 ratio with histone H1 by 48 h of culture. Standard amino acid analysis and NMR show that P2 is more HMG-like than H1-like and CD measurements demonstrated that P2 contains only 5% secondary structure in its maximally structured state and is, therefore, essentially unstructured under in vivo conditions. Also possible clustering of acidic residues is detected using CD and may be of functional significance. Analysis of post-translational modification of P2 shows that it is phosphorylated at up to three sites as isolated from immature spherules. The relationship of P2 to the HMG family of proteins and AS-2 is discussed.