The placental members of the prolactin-GH-placental lactogen (PL) gene family of the mouse include mPL-I, mPL-II, proliferin (PLF) and proliferin-related protein (PRP). The aim of the present study was to assess the effects of tumour necrosis factor-alpha (TNF-alpha) on the secretion of these proteins in primary cultures of placental cells from days 7, 9 and 12 of pregnancy. The effects of epidermal growth factor (EGF) on the secretion of PLF and PRP were also determined. EGF has previously been shown to stimulate mPL-I and inhibit mPL-II secretion. Incubation of placental cells from day 7 of pregnancy for 5 days with 10 nmol human (h)TNF-alpha/l did not affect the mPL-II concentration of the medium, but similar treatment of cells from days 9 or 12 of pregnancy resulted in a significant reduction in the mPL-II concentration of the medium by the second or third day of culture. The intracellular concentration of mPL-II, the number of cells that released mPL-II as assessed by reverse haemolytic plaque assay, and steady-state levels of mPL-II mRNA as assessed by Northern analysis were also reduced by hTNF-alpha treatment. The lowest concentration of hTNF-alpha that significantly inhibited mPL-II secretion by cells from day 12 of pregnancy was 0.01 nmol/l. hTNF-alpha treatment did not affect the secretion of mPL-I, PLF or PRP, as assessed by the concentrations of these proteins in the medium during a 5-day incubation. Incubation of the cells with 20 ng EGF/ml also did not affect the PLF or PRP concentration of the medium during 5 days of culture. To determine whether the effect of hTNF-alpha on mPL-II secretion was mediated by interleukin-6 (IL-6), the IL-6 concentration of the medium of control and hTNF-alpha-treated cells was determined. Bioactive and immunoreactive IL-6 could not be detected in medium from either treatment group. The presence of binding sites for hTNF-alpha was assessed in cells from day 12 of pregnancy. Scatchard analysis detected a single class of binding sites having a Kd of 1.61 +/- 0.34 nmol/l, with about 1350 sites per cell. The results of this study demonstrate that hTNF-alpha inhibits the secretion of mPL-II by placental cells from days 9 and 12 of pregnancy, suggesting that TNF-alpha may be one of the factors that regulate the production of this hormone in vivo.