Cathelicidins are antimicrobial peptides, stored by mammalian leukocytes, showing an antimicrobial activity against bacteria, fungi, protozoa and enveloped viruses. In accordance with other authors, we reported in a previous study that the protegrin-1 (PG-1), at 80 mgmL1, inhibited the in vitro growth of Chlamydia trachomatis serovars D, H and L2; however, we observed an increased infectivity of some animal chlamydial species after their treatment with the same PG-1 concentration. In this study, the treatment of LLC-MK2 cells with PG-1 before chlamydial infection resulted in an increased infectivity of Chlamydophila abortus probably due to their easier entry into the host cells, whereas no increase in S26/3 infectivity was detected in LLC-MK2 cells treated with PG-1 postchlamydial infection.