The effects of chloramphenicol (CAP) and thiamphenicol (TAP) on the outcome of Chlamydiapsittaci infection in chick embryos were compared. CAP administered along with Chlamydia reduced embryo mortality rates but showed no appreciable effects when its injection was delayed. On the contrary, TAP caused a high rate of embryo survival in both experimental situations. Statistical analysis of the results showed that differences in the survival rates following CAP and TAP administration are significant. Metabolic pathways in chick embryos of the antibiotics assayed differed remarkably in that CAP undergoes a quicker inactivation, which could even justify the better activity showed by TAP.