In order to obtain a transgenic mouse model of sickle cell disease, we have synthesized a novel human beta‐globin gene, beta SAD, designed to increase the polymerization of the transgenic human hemoglobin S (Hb S) in vivo. beta SAD (beta S‐Antilles‐D Punjab) includes the beta 6Val substitution of the beta S chain, as well as two other mutations, Antilles (beta 23Ile) and D Punjab (beta 121Gln) each of which promotes the polymerization of Hb S in human. The beta SAD gene and the human alpha 2‐globin gene, each linked to the beta‐globin locus control region (LCR) were co‐introduced into the mouse germ line. In one of the five transgenic lines obtained, SAD‐1, red blood cells contained 19% human Hb SAD (alpha 2 human 1 beta 2SAD) and mouse‐human hybrids in addition to mouse hemoglobin. Adult SAD‐1 transgenic mice were not anemic but had some abnormal features of erythrocytes and slightly enlarged spleens. Their erythrocytes displayed sickling upon deoxygenation in vitro. SAD‐1 neonates were anemic and many did not survive. In order to generate adult mice with a more severe sickle cell syndrome, crosses between the SAD progeny and homozygous for beta‐thalassemic mice were performed. Hemoglobin SAD was increased to 26% in beta‐thal/SAD‐1 mice which exhibited: (i) abnormal erythrocytes with regard to shape and density; (ii) an enlarged spleen and a high reticulocyte count indicating an increased erythropoiesis; (iii) mortality upon hypoxia; (iv) polymerization of hemolysate similar to that obtained in human homozygous sickle cell disease; and (v) anemia and mortality during development.