The extent of the contribution of genetic and environmental factors to the variation in levels of serum immunoglobulin G, A, M and E was studied among families of Arabs and Kurds of Iraq. Heritability (h2) values were calculated from offspring-parent, offspring-mid-parent and sib-sib comparisons and were all positive and moderate. All values were significant for all four immunoglobulins, being generally lower in Arabs than in Kurds. The h2 estimate based on offspring-parent and offspring-mid-parent were generally lower than those based on sib-sib comparisons, suggesting that these values were inflated by the common environment of full sibs. Genetic correlations between IgG--IgE and IgG--IgA were relatively high, reflecting a common regulation in the level of these immunoglobulins. IgM showed low genetic correlation with other immunoglobulins. All environment correlations were positive, and rather higher in the Kurds than the Arabs, indicating that environmental factors influence the level of all four classes of immunoglobulins.