Granulocyte macrophage-colony stimulating factor (GM-CSF) has hormone-like effects on female reproductive systems. Recent evidence has suggested that GM-CSF also might be important to male testicular function. It is possible, however, that most sources of GM-CSF might not be able to reach the testis, since testes are shielded from contact with the general circulation by vascular and Sertoli cell barriers, which together comprise the blood-testis barrier (BTB). We used a sensitive in vivo method to determine whether blood-borne GM-CSF crossed the BTB in mice. 125I-GM-CSF was found to cross the BTB, showing a unidirectional influx constant (Ki) of 1.45 x 10(-3) ml/g-min, nine times faster than the influx rate of the control substance, 99mTc-albumin. HPLC analysis confirmed the presence of intact 125I-GM-CSF in the testis after peripheral injection. More 125I-GMCSF than 99mTc-albumin crossed both the vascular barrier, to enter the testicular interstitial fluid, and the Sertoli cell barrier, to enter the seminiferous tubule fluid. Coinjection of unlabeled GM-CSF significantly reduced the passage rate of labeled cytokine across the BTB, demonstrating that passage was mediated by a saturable system and suggesting the presence of a transport or facilitated diffusion system for GM-CSF. In summary, GM-CSF passes intact from blood to testis by means of a saturable mechanism.