The effect of tetracycline (TC) on the behavior of TiO2 nanoparticles (NPs) in solution is investigated. The results illustrate that TC molecules do not exhibit an obvious effect on the aggregation of TiO2 NPs under circumneutral conditions (pH of approximately 5.5). However, the TC molecules exhibit a remarkable effect on the behavior of TiO2 NPs when the environmental factors (such as pH, ionic strength, and humic acid; HA) are adjusted. In the pH range from 3.0 to 10.0, the zeta potentials of NPs become more positively charged after TC adsorption. The point of zero charge of the TiO2 NPs increases from 6.5 to 7.5. The stability of the TiO2 NPs is improved after TC adsorption in different salt solutions. The critical coagulation concentration (CCC) increases from 10.57 mmol/L to 17.21 mmol/L, 4.95 mmol/L to 9.73 mmol/L, and 0.23-1.67 mmol/L in the presence of NaCI, CaCl2, and Na2SO4 (where Cl- and SO42- are the counter-ions), respectively. Under different HA concentrations, the C-HAPZC (the HA concentration that causes the zeta potential of NPs to be zero) increases from 0.5 mg/L to 1.3 mg/L after TC adsorption. The sizes of the TiO2 NP aggregates are larger than those without TC adsorption. All of these results are due to an increase in the surface charge of the TiO2 NPs after TC adsorption. The adsorption of H+ during TC adsorption causes protonation of the dimethylamine group on the TC molecules. (C) 2017 Elsevier B.V. All rights reserved.