To investigate factors controlling spermatogonial proliferation, we used premeiotic (PrM) spermatocysts (stage-synchronized germ cell/Sertoli cell clones) derived from the testis of the dogfish shark (Squalus acanthias) as an in vitro test system to estimate DNA synthesis by [3H]thymidine incorporation. Coculture of PrM spermatocysts with spermatocysts of the same (PrM) or more advanced stages (M, meiotic; PoM, postmeiotic) revealed the presence of an increasing gradient of inhibitory bioactivity from immature to mature stages (PoM > M > PrM). An even more potent and effective inhibition was detected when PrM spermatocysts were cocultured with equivalent amounts of epigonal organ, a lymphomyeloid tissue encapsulating the testis adjacent to the mature (PoM) region and immediately upstream in the vascular pathway. Inhibitory bioactivity also was present in spent media from cultured epigonal fragments and in cytosolic subfractions of epigonal homogenates but was undetectable in epididymis, muscle, serum, and red blood cells. Lower but significant inhibition was obtained with the white blood cell fraction of peripheral blood in one of two experiments. Effects of the epigonal growth-inhibitory factor (EGIF) were dose- and time-dependent, had a short response latency (3 h), were completely reversible (< 24 h after washout), and counteracted but did not block the stimulatory effects of insulin on [3H]thymidine incorporation by PrM spermatocysts. EGIF was equally inhibitory when tested on each of five PrM substages (stem cells-->preleptotene). Analysis of the entire series of experiments showed that testis-derived inhibitory activity varied seasonally, with maximum effectiveness correlated with periods of spermatogenic inactivity.(ABSTRACT TRUNCATED AT 250 WORDS)