Ab levels in the genital tract may be important in fertility and in preventing sexually transmitted diseases. In this study, 125I-labeled polymer or monomer mAb IgA (C4pIgA or C4mIgA) and IgG2b (C4IgG) to murine lactate dehydrogenase C4 and a polymer mAb IgA (nplgA) not cross-reacting with mouse sperm were intravenously injected into BALB/c mice, and the relative distribution of these Abs was determined. Polymer IgA was transported much more efficiently into the genital tract, trachea, and duodenum of both sexes than C4IgG and C4 mIgA (p < 0.01). The transport of polymer IgA (C4pIgA and npIgA) into the male genital tract greatly increased following orchiectomy (p < 0.01); this change was not affected by testosterone, suggesting that the unknown regulatory factor(s) from the testis may suppress polymer IgA transport. However, the transport of polymer IgA into female genital tissues was significantly decreased by ovariectomy (p < 0.01); this decline can be rectified by beta-estradiol but not progesterone treatment, suggesting that estradiol may stimulate polymer IgA transport. Furthermore, the transport of C4IgG into tissues of the Fallopian tubes and the uterus was significantly decreased by treatment with progesterone (p < 0.01). Together, these findings indicate that serum polymer IgA can be transported selectively into the genital tracts of both sexes, that this transport is strongly under the control of gonads, and that transport of IgG into the Fallopian tubes and uterus is down-regulated by progesterone.