Granulosa cells from the first (Fl), third (F3) and fifth and sixth (F5–6) preovulatory follicles and the small yellow follicles (SYFs; diameter 6–8 mm) were cultured for 21 h in the absence and presence of murine and human epidermal growth factors, fibroblast growth factor, transforming growth factors α and β-I (TGFα, TGFβ), platelet-derived growth factor and insulin-like growth factor-I at concentrations of 0·1–100 ng/ml. Plasminogen activator (PA) activities in the cell (PAc) and in the medium (PAm) were measured by fibrinolysis and fibrin overlay methods. Basal PAc and PAm activities were highest in cell cultures from the less mature follicles (F5–6 and SYF) and decreased as the follicles matured (F3>F1). PAc activity was greater than PAm activity, irrespective of the stage of follicular development. All growth factors examined at the 100 ng/ml level were effective in increasing PAc and PAm activities in cultures of granulosa cells from Fl follicles. However, only TGFα was able to increase PA activities at lower concentrations. The stimulation of the PA activities of granulosa cells from F3 follicles was inconsistent. None of the growth factors significantly increased PA activities in granulosa cells from F5–6 follicles and SYFs, as determined by fibrinolysis. The major PAc and PAm species (characterized by fibrin overlay) had a molecular mass of about 35 kDa, which is characteristic of the urokinase type. Both assay methods detected a stimulatory effect of the growth factors on PA activities in the granulosa cells from Fl follicles. However, an increase in PA activities in cells from F3 and F5–6 follicles and SYFs was indicated only after fibrin overlay analysis. Tritiated thymidine was incorporated into the DNA of granulosa cells at all stages of follicular development and was enhanced by all growth factors, although TGFα and TGFβ were the most effective and had a ranked order of activity: F3, F5–6>F1, SYF. The present findings show that, of the growth factors examined, TGFα may be an effective regulator of PA activity in avian granulosa cells during follicular development, in addition to its observed mitogenic action.