Female Beagles were inoculated intradermally with a sublethal dose of Rickettsia rickettsii and R montana. Three dogs (group 1) were inoculated with 2 X 10(2) plaque-forming units (PFU) of R rickettsia and were treated with tetracycline beginning on postinoculation day (PID) 12; 3 dogs (group 2) were inoculated with 2 X 10(2) PFU of R rickettsii but were not treated; 3 dogs (group 3) were inoculated with 2 X 10(2) PFU of R montana. Group-3 dogs failed to seroconvert and were inoculated a second time on PID 68. Groups 1 and 2 dogs inoculated with R rickettsii became depressed and developed occasional inappetence, fever, hematochezia, and ocular lesions. These dogs had a decrease in PCV and RBC count, an initial decrease in WBC count followed by leukocytosis, and a decrease in platelet count. Group-3 dogs inoculated with R montana remained healthy. After R rickettsii inoculation, the serologic response to spotted fever group (SFG) rickettsial antigens (R rickettsii, R rhipicephali, R montana, and R bellii) was similar. The antibody response to R rickettsii was first detected on PID 9, with peak titers reached by PID 20. Serum titers to R rickettsii remained stable or decreased one dilution through PID 120. Of 4 SFG rickettsial antigens, the highest serologic response was to R rickettsii. A cross-reacting antibody response with R rhipicephali and R montana was nearly identical and was only slightly less than the response to R rickettsii. Cross-reacting antibodies to R belli were of lower mean titer and of shorter duration than were cross-reacting antibodies to other SFG rickettsiae.(ABSTRACT TRUNCATED AT 250 WORDS)