Ceruloplasmin, the blue copper-protein of the blood plasma, is believed by some workers to be involved in the metabolic management of 5-hydroxytryptamine (serotonin) during pregnancy. 5-Hydroxytryptamine is abortifacient in experimental animals. By some authors, a role has been suggested for it in human abortion. The plasma concentrations of 5-hydroxytryptamine, copper and ceruloplasmin were measured in non-pregnant women, in normal early pregnancy and in cases of spontaneous abortion. Compared with normal early pregnancy, cases of inevitable abortion show lower values for both plasma copper and ceruloplasmin and higher values for plasma 5-hydroxytryptamine. The possible implications of these findings are discussed in view of the alterations in ceruloplasmin values in pregnancy and in the light of what is known of the pharmacology and metabolism of 5-hydroxytryptamine. It it believed that these alterations in plasma copper, ceruloplasmin and 5-hydroxytryptamine in cases of inevitable abortion are the effect, rather than the cause, of abortion.