Ten different brands of tetracycline were given to ten medical students in a cross-over study. A single dose of 500 mg was given, blood samples were taken 1,3,6, and 12 h later, and 24 h urines were collected. Peak serum levels of different preparations varied from 2.91±0.45 µg/ml to 4.32±0.31 µg/ml (mean±S.E.M.). According to both the area under the serum concentration-time curve as well as the total amount excreted in 24 h, the preparations could be divided into two poorly defined groups. Six preparations did not differ markedly from each other. Four other preparations gave lower values and differed significantly from two or more of the preparations of the first group. The smallest area and the lowest excretions were 68% and 74% of the largest area and the highest excretions, respectively. Although all the preparations gave adequate serum concentrations, four of them showed significantly lower absorptions — about 70 to 80% — than did the best brands.