In a papillary thyroid carcinoma cell line, TPC-1, we found transcripts hybridizing to ret proto-oncogene (proto-ret) cDNA probes. The transcripts hybridized to the probes encoding the kinase domain but not to those of the transmembrane and extracellular domains of proto-ret. The sizes of the main transcripts in TPC-1 were aberrant, being 2.0, 2.5, 4.0 and 5.0 kb. In the neuroblastoma cell lines, the transcripts were 3.9, 4.5, 6.0 and 7.0 kb, which have been proved to be generated by alternative splicing and polyadenylation from the non-altered proto-ret oncogene. All of the four transcripts in TPC-1 are about 2 kb smaller than the corresponding ones in the neuroblastoma. From the length of the transcripts, it is suspected that the transcripts in TPC-1 are a rearranged form of proto-ret. This is the first report describing the aberrant transcripts of proto-ret in a human tumor.