Comparisons of the inhibitory activities of different tetracyclines have been reported for Plasmodium falciparum but no other parasites. The in vitro response of the intestinal parasite Giardia lamblia to six tetracyclines in current use was determined. In addition, the experimental drug thiacycline (EMD 33,330) was evaluated. Three groups were discerned, with representative 50 and 90% inhibitory concentrations of, respectively, 36 and 130 (tetracycline), 6.4 and 22 (doxycycline), and 1.8 and 3.4 (thiacycline) micrograms/ml. These dramatic differences in activity correlate with increased lipophilicity.