The effect of Eimeria tenella infection on the Salmonella enteritidis infection in chickens was investigated in three experiments. Each experiment consisted of an uninfected control, birds infected with E. tenella, birds infected with S. enteritidis, and birds infected with a combination of E. tenella and S. enteritidis. A dose of 2 x 10(4) sporulated oocysts of E. tenella was given to each chicken. In Experiment 1, each chicken was infected with 10(5) cfu/d of S. enteritidis for 3 consecutive d starting 1 d after coccidia exposure and necropsied 4, 7, 10, and 14 d after coccidial infection. In Experiments 2 and 3, each bird was infected with 10(2) cfu/d and 10(1) cfu/d of S. enteritidis respectively, for 2 d beginning 4 d after coccidia exposure, and were killed 7, 10, and 14 d postinfection with E. tenella. Results showed that interaction between S. enteritidis and E. tenella was significant in Experiments 1 and 2, but not in Experiment 3, manifesting that the cecal Salmonella population were significantly increased by coccidial infection in birds killed 4, 10, or 14 d after coccidia exposure in Experiment 1 and in birds killed 14 d after coccidial infection in Experiment 2. The dose size of S. enteritidis was an important factor for the synergistic relationship between S. enteritidis and E. tenella. Frequency of recovery of S. enteritidis in liver and spleen was not increased by E. tenella infection.