A series of both aliphatic and aromatic amides and aromatic amidines derived from 2-amino-1,10-phenanthroline (3) according to the Topliss scheme were synthesized and subsequently tested for antimycoplasmal potency. Although the compounds themselves showed no activity, in the presence of a nontoxic copper concentration of 40 microM all compounds appeared to be very active against Mycoplasma gallisepticum K154. The most active compounds were found in the amide series and show growth inhibition in the nanomolar range. These compounds are 4 times more active than tylosin, a macrolide antibiotic, which is used therapeutically in veterinary practice. In the presence of copper, amides derived from 3 are more active than corresponding amidines. Increased activity following derivatization of 3 may be due to the presence of a third coordination site for copper in the title compounds. Evaluation of biological data revealed that antimycoplasmal activity of amides derived from 3 is dependent on lipophilicity. For these amides a good linear correlation was found between antimycoplasmal activity and hydrophobic fragmental values for substituents considered. This quantitative structure-activity relationship study indicated that antimycoplasmal activity was increased upon a decrease of these hydrophobic fragmental values.