Four in vitro precipitation methods were tested and evaluated using flavopiridol and diazepam formulations. The methods include static serial dilution, dynamic injection, and dropwise addition with and without stirring. The results generated from these methods are comparable and complementary. The static serial dilution method is most effective in quantifying the amount of precipitation and more descriptive of the formation and redissolution of the precipitate than the others. The dynamic injection method, however, has its merit in more realistically simulating the physiological environment of drug-blood interaction near the injection sites.