Epidemiological (cohort follow-up) and laboratory techniques studies were done to validate a programme of chemoprevention of canine monocytic ehrlichiosis (CME) with this molecule. 614 dogs returning to France after having spent at least 4 months in a CME-endemic area (Africa, Guyana, Middle-East, etc.) were the object of systematic serological testing by indirect immunofluorescence (IFA). The dogs were given 100 mg of doxycycline per os daily for chemoprevention of CME. In addition, HPLC (high performance liquid chromatography) was used to determine plasma levels of doxycycline in 124 of the dogs. The CEM mortality and morbidity rates for the 614 dogs in the chemoprevention programme were nil. The seroconversion rate was 4% (24/614). Seropositive dogs (low titres) were asymptomatic and generally became seronegative after treatment. A study done on 10 dogs shows that doxycyclinaemia was 1.2 (0.94-1.53) microg/ml 2 h after the drug had been administered. After 24 h, the residual concentration was 0.34 (0.26-0.44) microg/ml. Blind doxycyclinaemia tests done on 110 dogs living in Africa (the results for four dogs were nil and therefore eliminated from the study) showed that the minimum observed concentration was always greater than 0.2 microg/ml. Given that, as concerns infection with Ehrlichia spp., the minimum inhibitory concentration of doxycycline is < or = 0.03 microg/ml, dogs receiving chemoprevention treatment should be protected.