The secretion of the ABH antigens in saliva was tested in indigenous individuals of several populations: Icelanders in Reykjavik and Husavik (northeastern Iceland), Aland Islanders, Finno-Ugrians (Finns, Finnish Lapps, Komi) and Eskimos (Augpilagtok, northwestern Greenland). The frequencies of ABH non-secretors among the Icelanders (28-36%) were among the highest ever noted in Europeans. Among Alanders and Swedes on the Finnish mainland the frequency (around 20%) was comparable to Swedish values but considerably higher than among Finns (13-14%). The values among northeastern Finns and Komi (about 9%) were intermediate between values among Lapps (below 5%) and Scandinavians (15-26%), excluding Icelanders (28-41%). The average frequency of non-secretors among Lapps in Finland (2.2 +/- 0.5%) was the lowest observed among white populations. Like many other arctic populations of the Mongolian race, the Greenland Eskimos had a very low frequency of non-secretors. It is probable that the non-secretor allele ABH*se was absent from the ancient Lapps and Greenland Eskimos but introduced by invading populations. It is concluded that the ABH*se allele frequencies vary much more among northern European populations than hitherto appreciated. Recent studies indicate that the non-secretor status of the ABH blood group substances in mucous body fluids is associated with pathological conditions of the mucous membranes of the embryologically related digestive and respiratory systems, particularly with duodenal ulcer and gastric (pre)malignancies but probably also with pulmonary dysfunction. In view of these disadvantages of the ABH non-secretor status the high frequency of ABH*se in Icelanders is a paradoxical phenomenon. The frequency of ABH non-secretors among the founders (Vikings) of Iceland may have been considerably higher than among the present populations in northwestern Europe. The increase in northwestern direction of the ABH*se allele frequencies supports this hypothesis; the dilution effect has not been as strong in Iceland as on the European continent.