Interactions between poly(ethyleneimine) (PEI) and porcine muscle lactate dehydrogenase (LDH) were studied using static and dynamic light scattering, and intrinsic fluorescence spectroscopy. Time-related aggregation of the enzyme was reduced in the presence of the polymer. PEI of molecular weights 2000 and 25000 were found to complex with the enzyme without significant change in the particle hydrodynamic radius, i.e., the polymer is bound in a flat conformation at the enzyme surface. Interactions with the high molecular weight PEI (2.6 x 10(6)), on the other hand, resulted in a particle of a size significantly larger than the polymer, indicating that several LDH molecules may bind within the large, branched polymer structure. Although it was not found possible to quantitatively determine the degree of binding, estimation of the binding constants by intrinsic fluorescence measurements revealed weak binding between the enzyme and the polymer.