In dairy goats, treatments associating a progestogen and the equine chorionic gonadotropin (eCG) are the easiest way to induce and synchronize estrus and ovulation, and to permit artificial insemination (AI) and/or out-of-season breeding. From the first treatment, injection of eCG induces, in some females, the production of anti-eCG antibodies (Abs) which will interfere with the effectiveness of subsequent treatments. These anti-eCG Abs delay the preovulatory LH surge and the ovulation time, leading to poor fertility of the treated females. Here, by in vitro bioassays, we show that anti-eCG Abs can positively or negatively modulate the LH and/or FSH bioactivities of eCG. Moreover, the modulation level of eCG bioactivity does not depend on the anti-eCG Ab affinity for eCG, as shown by surface plasmon resonance technology. The specificity of anti-eCG Abs tested by competitive ELISA highlighted the importance of a glycan environment in the recognition mechanism, especially the sialic acids specific to eCG. The different effects of anti-eCG Abs on eCG bioactivities could be explained by two hypotheses. First, a steric hindrance preventing the interaction of eCG to its receptors would explain the inhibitory effect of some anti-eCG Abs; second, a conformational change of eCG by anti-eCG Abs could induce an inhibition or a potentiation of the eCG bioactivities. It is significant that these modulations of eCG bioactivities by anti-eCG Abs impact mainly on the FSH bioactivity of eCG, which is essential for ovarian stimulation and subsequent fertility after treatment and AI, and to a lesser extent on the LH bioactivity.