Serum alphafoetoprotein (AFP) and serum alpha-1 antitrypsin (AAT) were determined in 24 patients with germ-cell neoplasms of the gonads and extragonadal sites and in two patients with hepatocellular carcinoma. In the majority of the patients serial determinations were performed. All seven patients with testicular seminoma and four patients without evidence of active disease had normal levels of serum AAT and AFP. The remaining 13 patients with germ-cell neoplasms had tumours containing endodermal sinus tumour (yolk-sac tumour) elemetns. All these 13 patients had elevated levels of serum AFP and the levels were high or very high in most cases. Nine of these 13 patients had raised serum AAT, although the elevation above normal levels was only slight in a number of cases. When serial determinations were performed serum AAT levels frequently followed the pattern of serum AFP levels, but the AAT levels were frequently within normal limits and therefore the interpretation of the results was difficult, and much less reliable as compared with those for serum AFP. The elevation of serum AAT levels following the recurrence of the tumour was found to occur much later and was much less marked than elevation of serum AFP, which occurred early, showed a large rise and was a reliable marker of tumour recurrence in patients with germ-cell neoplasms containing endodermal sinus tumour elements. It is therefore considered that, although there is good evidence that serum AAT is produced by endodermal sinus tumour elements, serum AAT is not a useful monitor of disease activity in these patients, especially when compared with serum AFP, the value of which is well recognized. Serum AAT may be a useful tumour marker in patients with hepatocellular carcinoma, and this aspect should be investigated further.