Abstract: Following oral administrations of a single dose of 150 mg roxithromycin dispersible tablet (formulation A, a new formulation for clinical trial) and conventional tablet (formulation B, purchased from the market) to each of 10 healthy male volunteers in a randomized crossover design, the plasma levels of active drug at different times were determined by a microbial assay with Micrococcus luteus CMCC (B) 28001 and the plasma concentration—time profiles were obtained. The T_max values of formulation A and B obtained were 1.7±0.9 and 3.7±1.6 h, the C_max values were 4.97±1.17 and 2.04±1.26 μg·ml^-1 and the AUC_0→∞ values were 62.2±11.9 and 35.0±16.9 μg·h·ml^-1, respectively. Surprisingly, the relative bioavailability of formulation B was found to be only 59.8%±32.6% (P<0.01). The low bioavailability of the latter formulation was attributed to poor release of roxithromycin in the gastric site, which was proposed as a key factor to influence drug absorption. Other related factors were also discussed.