Serine/threonine kinase transmembrane proteins are a new family of growth factor signal transducers that includes several isoforms of the activin type II receptor and the type II receptor for transforming growth factor-beta. In an effort to clone the receptor for Mullerian inhibiting substance, a member of the transforming growth factor-beta superfamily, oligonucleotide primers designed from conserved regions of these receptors' kinase domains were used for PCR amplification of fetal rat urogenital ridge cDNA. We isolated four novel receptors in this manner (designated R1-R4), each of which has structural features of the previously cloned kinases, including a small extracellular ligand-binding domain, a single hydrophobic transmembrane domain, and an intracellular serine/threonine kinase domain. In addition, each has characteristic kinase subdomains and conserved serine/threonine kinase sequences found in this family. Northern analysis revealed mRNA expression of R1-R4 in several tissues, including fetal urogenital ridge, testis, and ovary, as well as brain and lung. In situ hybridization further localized R1 to mesenchyme of the 14.5 to 15-day fetal rat Mullerian duct and to oocytes of preantral and antral follicles, sites that are consistent with the predicted localization of Mullerian inhibiting substance receptor. In addition, R2 localized specifically to seminiferous tubules of the postnatal testis. These newest members of the activin and transforming growth factor-beta type II receptor family should help define the molecular mechanisms by which this ligand superfamily affects cell growth and differentiation via membrane phosphorylation.