Targeted disruption of the mouse activin receptor type IIB gene (Acvr2b) results in abnormal left-right (LR) axis development among Acvr2b-/- homozygotes [Oh and Li, 1997: Genes Dev 11:1812-1826]. The resulting malformations include atrial and ventricular septal defects, right-sided morphology of the left atrium and left lung, and spleen hypoplasia. Based on these results, we hypothesized that mutations in the type IIB activin receptor gene are associated with some cases of LR axis malformations in humans. We report here characterization of the ACVR2B genomic structure, analysis of ACVR2B splice variants, and screening for ACVR2B mutations among 112 sporadic and 14 familial cases of LR axis malformations. Two missense substitutions have been identified, one of which appears in two unrelated individuals. Neither of these nucleotide changes has been found in 200 control chromosomes. We conclude that ACVR2B mutations are present only rarely among human LR axis malformation cases.