The attachment of Chlamydia psittaci, strain 6BC, to formaldehyde-fixed and unfixed L cells was studied. Cations were found to be required for attachment to both fixed and unfixed cells. The requirement for cations was largely eliminated when the net negative surface charge on fixed cells was reduced. A high concentration of sodium chloride (0.5 M) prevented binding and removed chlamydiae which were attached to fixed and unfixed cells, whereas non-ionic detergents had no effect on attachment of C. psittaci to fixed cells. The effect of various modifications of C. psittaci and L cell surfaces on attachment was also determined. Of the treatments tested, only trypsinization and periodate oxidation of L cells and acetic anhydride, heat and periodate treatments of C. psittaci reduced binding. Various lectins and high concentrations of neutral sugars had no effect on attachment, whereas, amino sugars and several organic amines inhibited attachment. These results suggest that the initial phase of attachment requires electrostatic interactions between host and parasite surfaces, and that amino and carbohydrate groups on the surface of C. psittaci and glycoproteins on the surface of L cells may be directly or indirectly required for attachment.