Specific, high-affinity binding sites for radiolabelled mouse epidermal growth factor (mEGF) were demonstrated in homogenates and membranes of ovine corpora lutea. Subcellular fractionation on sucrose density gradients demonstrated that luteal EGF receptors were associated with fractions enriched in cell surface-membrane markers. Binding of 125I-labelled mEGF to ovine luteal EGF receptors was dependent on the pH, temperature and duration of incubation, and on the concentration of metal ions present in the incubation medium. Unlabelled mEGF and human EGF (urogastrone) competed for the binding of radiolabelled mEGF to ovine luteal homogenates at low doses (half-maximal inhibition of binding (IC50) at 2-3 nmol/l). Transforming growth factor-alpha also competed for mEGF-binding sites (IC50, 4-10 nmol/l), but a range of peptides, growth factors and protein hormones were ineffective at much higher concentrations. Concave Scatchard plots for 125I-labelled EGF binding and Hill coefficients of < 1 for displacement radiolabelled EGF suggested negative co-operativity of binding sites, and dilution at equilibrium accelerated the rate of dissociation of 125I-labelled EGF from human placental (but not from ovine luteal) receptors. Specific EGF-binding sites were also demonstrated in rat and rabbit placental homogenates, and in luteal homogenates of the pig. Luteal concentrations of EGF receptors appeared to be reduced significantly during early pregnancy in both the pig and sheep.