A recombinant fowlpox virus (rFPV) expressing the Eimeria tenella rhomboid gene was constructed and its protective efficacy against homologous infection in chickens determined. Three-day-old-specific pathogen free (SPF) chickens were immunized s.c. with 10(2) plaque forming units (PFU), 10(4) PFU, or 10(6) PFU of rFPV-rhomboid, and challenged with 5x10(4) homologous sporulated oocysts 14 days post-immunization (p.i.). The specific antibody response and lymphocyte proliferation were measured 1, 2, 3 and 4 weeks p.i. Oocyst output, body weight gains and lesion scores were measured to evaluate the protective effects of immunization. rFPV-rhomboid elicited a specific humoral immune response and stimulated proliferation of peripheral blood lymphocytes. The lesion scores in groups vaccinated with rFPV-rhomboid were significantly higher than in other groups. At the same time, rFPV-rhomboid improved body weight significantly compared with other groups. Immunization with rFPV-rhomboid reduced oocyst shedding significantly, resulting in a protection rate of 39.6%, 41.1% or 41.7% given a dose of 10(2) PFU, 10(4) PFU, or 10(6) PFU of rFPV-rhomboid, respectively. These results indicated that rFPV can induce immune responses and offer partial protection of chickens against E. tenella challenge.