Three brands of acetaminophen tablets marketed in Turkey were evaluated for their in vitro properties, and their in vivo bioavailabilities were determined in comparison to a standard acetaminophen solution containing an equivalent amount of the drug. The total amount of acetaminophen in urine or in tablets was determined by the colorimetric method of Welch and Conney [1965]. No significant difference between the manufacturing parameters, namely the assay, hardness, diameter and thickness of the brands was observed except that the coefficients of variation for brand A were greater than the other two brands. Brand A also disintegrated more slowly and its dissolution time (80% of the drug dissolved in 45 min) did not meet the requirements of USP XX. In vivo bioavailability studies were carried out in six healthy volunteers who received the tablets or the standard solution in a single dose, cross-over study. The urine was collected for 24 h and the total acetaminophen excreted was determined according to the method of Welch and Conney. The relative bioavailabilities for Brands A, B and C were found to be 97.87, 95.41 and 98.78%, respectively, with respect to the solution. The differences were not statistically significant and it can be concluded that all tablets were rapidly and almost completely absorbed. This study also demonstrates that there is not always a good correlation between the in vitro disintegration and dissolution tests and the in vivo bioavailability of a drug.