Calves vaccinated with Anaplasma centrale were treated with 20 mg/kg of long-acting oxytetracycline (OTC/LA) before or simultaneously with vaccination or up to seven months later. Of 40 animals given one or two of OTC/LA from 3 to 13 days before vaccination, 23 become patent after vaccination, with an average prepatent period almost twice as long as that in non-treated vaccinated controls. Upon challenge with 2 x 10(8) A. centrale per dose all 17 previously non-patent calves showed average maximum parasitemias of 2 to 3.8%. Out of 30 calves treated with two to four doses of OTC/LA from one to four weeks after vaccination, 29 remained negative for A. centrale and reacted to challenge infection with average maximum parasitemias of 6.9-7.8%. Five out of 10 calves receiving OTC/LA simultaneously with the vaccination, and all of a separate group of 10 calves treated with a single dose seven days after vaccination, become patent an average of 51.6 and 63.5 d, respectively, after vaccination. Upon challenge, the five previously non-patent calves showed an average of 5.2% maximum parasitemia. In all groups, only rare parasites were seen in previously patent calves after challenge. Thirty calves treated with 2-4 doses of OTC/LA about six months after vaccination showed no or only a few parasites upon challenge. The above results show that treatment with single or multiple doses of OTC/LA a few weeks before or after administration of live A. centrale vaccine can interfere with elaboration of immunity.