Abstract: A method for modifiying a liquid assay reagent to provide prolonged homogeneity thereof, particularly where the liquid assay reagent comprises microparticles for performing a heterogeneous immunoassay, is provided wherein the addition of an inert material to a liquid assay reagent achieves neutral density to thereby prolong the homogeneity thereof for extended periods of time. A method for the automated agitation of assay reagents to maintain the homogeneity thereof with an automated, continuous and random access analytical instrument is also provided. The automated mixing is accomplished by a back and forth motion of a carousel onto which assay reagent containers or packs are mounted with asymmetric pauses which can be completed within a short period of time. The carousel acceleration, velocity, distance moved, and pause-asymmetry are optimized to provide rapid assay reagent resuspension without foaming or bubble formation.