It has been possible to prepare electrostatically stabilized aqueous dispersions of lipid A (diphosphate) particles of low polydispersity at low ionic strength (1-10 mM NaCl) over a range of volume fractions of 1.5 x 10(-4) < 0 < 5.75 x 10(-4) (25 degreesC). These suspensions have been characterized by transmission electron microscopy, light scattering, osmotic pressure measurements, and small-angle X-ray scattering experiments at 25 degreesC. All four measurements yielded independent values for particle sizes, weighted-average molecular weights, number-average molecular weights, and particle surface charge. The mean values obtained are (R) over bar = 37.59+/-0.75 nm, (R) over bar (g) = 24.89+/-0.88 nm, (M) over bar (w) = (10.55+/-0.78) x 10(6) g/mol, Mn = (9.81+/-0.90) x 10(6) g/mol, and the effective surface charge Z* = (756+/-85). Very good experimental agreement is found for the directly measured osmotic pressure values and those determined from light scattering and small-angle X-ray scattering measurements as a function of volume fraction, phi, by applying liquid-state theory models. Using the particle parameters for the lipid A (diphosphate) system as determined, the scattering functions and the osmotic pressures can be compared as a function of volume fraction with no adjustable parameters. The ordering of lipid A in solution revealed a body-centered cubic (bcc) type lattice (a = 36.14 nm) at volume fractions of 3.75 x 10(-4) < phi < 4.15 x 10(-4), whereas at volume fractions of 4.15 x 10(-4) < (P < 5.75 x 10(-4) in the presence of 1.0 mM NaCl a face-centered cubic (fcc) lattice type (a = 57.25 nm) was observed. Small-angle X-ray scattering experiments also indicate the presence of long-ranged order at 1.0 mM or at 10.0 mM NaCl for lipid A dispersions of 3.75 x 10(-4) < phi < 5.75 x 10(-4).