The influence of milk protein emulsifying agents on the characteristics, particularly drug release, of polylactide microspheres was investigated. Diltiazem loaded polylactide (PL) microspheres were successfully prepared using the dairy proteins, sodium casinate (SC) and whey protein isolate (WPI) as the emulsifying agents. Microspheres were characerized in terms of microsphere yield, electron microscopy, particle size, drug loading, DSC and XRD analysis and drug release. The yields of microspheres obtained were 53-63% and were independent of the emulsifying agent used. SEM revealed that, regardless of the emulsifying agent employed, the microspheres were of good sphericity, but the surface appearance of the microspheres was not the same in all cases. The milk proteins resulted in microspheres approximately half the size of those obtained with methylcellulose (MC). Significant differences in drug loading were observed between the three emulgents, the MC systems giving the highest values. Release profiles were sigmoidal in shape and were well fitted to the equation ln(x/1 - x = k . t - k . t(max), reflecting degradation controlled drug release. The parameter k increased with drug loading, while tmax decreased. The relationships between the release parameters [P. k and t(max))] and loading (L) could be quantified by equations of the form P = a . L-N, N being negative in the case of tmax. Apart from the effect on loading efficiency, neither SC nor WPI appeared to significantly alter drug release. The quantitative relationships observed in this study may have more general application in quantifying drug release from drug-polymer composites at low loadings where polymer degradation controls drug release.