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The proliferative activity has been evaluated in 55 cases of seminoma and 36 cases of non-seminomatous germ cell tumors of the testis (NSGCT) in clinical stage I from patients alive and well within 5 years, by a semi-quantitative image analysis method of determination of the percent MIB-1 positive nuclear areas, using a Leica Quantimet 500+ analyzer. Significant differences of growth fraction have been detected between classical seminomas and seminomas showing more than 3 mitoses/1HPF (anaplastic seminoma)-mean of MIB-1 positivity in classic seminoma 23.8%, versus 43.9% in anaplastic seminoma; confirming this value of cut-off in distinguishing two classes of cases with different growth fractions, but without any difference as far as prognosis is concerned. The non seminoma- germ cell tumours (NSGCT) in toto display a growth fraction significantly higher than that of seminomas (mean value of MIB-1 positivity: 26.3% in seminoma versus 40.4% in NSGCT), but absolute values of positivity as high as those obtained by others by subjective evaluation have not been observed. A higher growth fraction in NSGCT is positively correlated with an embryonal carcinoma component as high as 80% of the tumor mass and/or with vascular invasion, but not with different histotypes. The proliferative activity doesn't stratify subgroups with different prognosis within the two groups of germinal tumours of the testis (seminomas and NSGCT) in our cases of clinical stage I testicular tumours.