Serum-alpha1-fetoprotein (A.F.P.) was assayed in 88 men with testicular teratomas and 34 with seminomas. 61% of teratoma patients with clinically evident disease had serum-A.F.P. greater than 25 ng/ml, whilst it is highly unlikely that any patient with a pure seminoma had a titre above this level. A.F.P. concentrations correlated with response to radiotherapy and chemotherapy and predicted relapse in some patients. Serum-A.F.P. should be measured in all patients with testicular tumours.