Sera from two Melanesian populations, one living at sea level and the other at 7000 feet, were examined for the incidence, strength and specificity of cold agglutinins. The cold auto‐agglutination incidence ranged from 48 to 90 per cent for the sea‐level group to 94 per cent for the highlanders. Titres of the sera ranged from 10 to 640. The specificity of the antibodies was in the main anti‐I, although four probable examples of anti‐AI, four examples of anti‐IT and two ‘non‐specific’cold agglutinins were found. Blood group A subjects in both populations had a higher incidence and titre of cold agglutinins than O subjects.The light chain types of 160 of 200 cold agglutinins tested were both k and λ, 24 were K only and 16 were λ only. Fingerprints of the H chains of six New Guinea cold agglutinins showed some peptide spots missing compared with normal IgM, but more spots were present than in a lymphoma cold agglutinin, suggesting that the cold agglutinins represented a heterogeneous population of antibodies.No obvious relationship was found between environmental factors and the occurrence of the cold agglutinins.

Genetic studies of 540 Paraguayan Indians from nine tribal groups and 51 Mennonites are presented for ABO, MNSs, P1, Rh, Kell, Lewis, Duffy, Diego; for serum immunoglobulins and haptoglobins, G6PD‐deficiency, and thalassemia trait. Group O gene frequencies for all Indian groups were 1.00; for r (cde), 0.00. Tapiete, Lengua, Toba, and Sanapana Rz (CDE) frequencies were among the highest ever reported. N frequencies were high for Ache Kwera (Guayaki), Lengua, Cheroti, Guarayu, Tapiete; N and s low for Ayore. MS frequencies were high for Sanapana, Lengua, Ayore; Ns for Tapiete. Diego was notably absent for Toba, Lengua, Guarayu, Tapiete, Ayore. Homogeneous frequencies for Fya (1.000) occurred among Guarayu and Tapiete, and for P1 among Guayaki. Inv(a) frequencies were low for Cheroti, Chulupi, Guayaki. Hp 1 among Guayaki (Ache Kwera 0.15) is lowest ever reported. G6PD deficiency and abnormal hemoglobins were uniformly absent from all groups. Mennonite results were homogeneous and point toward Dutch origins. Differences among groups studied, and between Paraguayan and other Amerinds emphasize importance of genetic drift and founder principle. Abandonment of their tribes by mixed‐blood offspring is partly responsible for apparent genetic purity and homogeneity of groups.

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