Sexual performance in adult male rats is highly sensitive to prenatal stress which can affect the functionality of the reproductive system and various brain structures involved in modulating sexual behavior. The immunomodulatory effect of mouse IgG on reproductive maturity in male offspring after LPS exposure in vivo and in vitro was studied. Prenatal IgG injection (20 µg/mouse) had a positive impact on the puberty of male mice whose mothers were exposed to LPS (100 µg/kg) on the 12th day of pregnancy. The numbers of Sertoli cells were increased, whereas the body weight and the number of symplastic spermatids were decreased in offspring as compared to LPS-treated animals. Besides, IgG had a positive effect on altered hormone levels: reduced estradiol level on the 5th and 14th postnatal days and increased testosterone level on the 30th postnatal day in blood that led to an increased number of mounting attempts in sexually mature males. The cAMP-dependent pathway may be involved in the regulation of the LPS-induced inflammation. IgG reduced the increased level of cAMP in mouse peritoneal macrophages activated by LPS in vitro. IgG is able to modulate inflammation processes but its exposure time is important.