Human macrophage inflammatory protein 1β (hMIP-1β), also known as Act-2, is a small protein that belongs to an extensive class of chemotactic cytokines. The high resolution solution structure of hMIP-1β has recently been determined by multidimensional NMR spectroscopy, revealing a dimeric molecule of 2 x 8 kDa. The fold of the monomer is very similar to that of the related cytokine, interleukin-8, which was solved previously. However, the mode of association whereby two monomers form a dimer is quite different. To maintain a sufficiently high concentration of soluble protein for NMR studies, hMIP-1β was dissolved at a pH of 2.5. During solubility trials, it was noticed that the hMIP-1β-containing solutions became birefringent at higher pHs, suggesting the formation of ordered light-scattering structures. We have investigated this phenomenon by negative staining electron microscopy.hMIP-1β was obtained from an expression vector, extracted from inclusion bodies, and purified as described. Protein solutions at 0.7 mg/ml, 100 mM NaCl, 20mM Na citrate, pH2.3, were dialyzed against 20mM ionic strength buffers at pH 4.5, 5.0 and 5.5 (sodium citrate), and pH 6.0 and 7.0 (sodium phosphate) at 4°C. Samples were withdrawn at various time-points, applied to glow-discharged carbon films, stained briefly (5s) with uranyl acetate, and examined in a Philips EM400RT microscope.