In insects, juvenile hormone (JH) decreases or has any effect upon the phenoloxidase (PO) ac-tivity, and favors or decreases the Antimicrobial Peptides (AMPs) expression. Although there is no information about the differential effect of such hormone, two possibilities are that it de-pends on (a) the immune marker recorded and (b) sexual differences. Here, three commonly used immune markers, Phenoloxidase (PO), hy-drogen peroxide (H 2 O 2), and lytic activity, were measured 3, 6 and 24 hours after administration of methoprene (JHa, an analog of juvenile hor-mone) in male and female monarch butterflies (Danaus plexippus). At 3 and 6 h post-JHa ad-ministration, the PO activity increased in fe-males but it only increased at 3 h in males, whereas H 2 O 2 levels increased only in females at 3 h. For the remaining times the JHa had a null effect on PO and H 2 O 2 . On the other hand, the JHa had a null effect for lytic activity in both sexes at 3, 6 and 24 h. To our knowledge, this is the first report of a positive effect of a JHa on PO and H 2 O 2 and suggests that this effect is sex dependent.