A cDNA clone coding for an oxytocin-like substance was prepared from a human placental cDNA library using oxytocin antiserum. The cDNA size was approximately 900 bp. A mammalian expression vector containing the cDNA was constructed and transfected into Chinese hamster ovary (CHO) cells. The expression of immunoreactivity to oxytocin antiserum was observed by radioimmunoassay. The cultured medium of the transfected cells was assayed for uterine muscle contractile bioactivity using a Magnus apparatus. Bioactivity was eliminated by incubation with oxytocin antiserum. Thus, the cDNA clone, screened with oxytocin antibody, is surmised to code for an oxytocin-like activity, but the nucleotide and amino acid sequences responsible for the biological activity remain to be clarified.