Coxiella burnetii, the causative agent of Q fever, survives and replicates in the acidic environment of monocytes/macrophages; hydroxychloroquine, through alkalinisation of the acidic vacuoles, is critical for the management of Q fever. In this study, a collection of C. burnetii strains isolated from human samples was tested to evaluate the in vitro minimum inhibitory concentrations (MICs) of doxycycline and hydroxychloroquine. Serial two-fold dilutions of doxycycline (0.25-8 mg/L) and hydroxychloroquine (0.25-4 mg/L) were added to C. burnetii-infected human embryonic lung (HEL) fibroblast cells after 48 h of incubation, in duplicate. DNA was detected by C. burnetii-specific semi-quantitative PCR with primers and probes designed for amplification of the IS1111 and IS30A spacers. A total of 29 C. burnetii isolates obtained from 29 patients were tested. Doxycycline MICs ranged from 0.25 mg/L to 0.5 mg/L and hydroxychloroquine MICs from 0.25 mg/L to >4 mg/L. Four C. burnetii stains had hydroxychloroquine MICs <= 1 mg/L. The concentration of hydroxychloroquine was associated with a significant decrease in C. burnetii DNA copies in HEL cells based on linear regression analysis (P = 0.01). Recommended serum concentrations of hydroxychloroquine significantly reduced the growth of C. burnetii. Moreover, some C. burnetii strains presented hydroxychloroquine MICs below the recommended serum concentrations, indicating that, for these cases, hydroxychloroquine treatment alone may even be effective. (C) 2017 Elsevier B. V. and International Society of Chemotherapy. All rights reserved.