Of the various cell types in the life cycle of Physarum polycephalum, only the growing plasmodium contains the unusual polyester beta-poly(L-malate). The nuclei exhibit large complexes of this polymer with nuclear proteins, among them DNA polymerase alpha, histones, and HMG-like proteins. The complexes are indicated by the results of size exclusion chromatography and chemical cross-linking with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide hydrochloride (EDC). After hydroxylaminolysis of the cross-linked polyester, the proteins are liberated and visualized on Western blots. The complexes of 1200-1400 kDa molecular mass exceed by far the size of free beta-poly(L-malate) and proteins. The observed variation in mass appears to be mainly a function of the kind and stoichiometry of the protein constituents and may explain the relatively high molecular mass in S phase and the low molecular mass during G2 phase of the mitotic cycle. The complexes are considerably stable at moderate ionic strength (100 mM KCl). Also, endogenous beta-poly(L-malate) does not exchange with added beta-[14C]poly(L-malate) during the lysis of the nuclei and the sample preparation. The complexes are dissociated at elevated concentrations of KCl, in the presence of spermine hydrochloride, or by treatment with DEAE/cellulose. Available evidence indicates that beta-poly(L-malate) may be involved in the maintenance of the plasmodial state of P. polycephalum.