A 3' region of a previously unknown retroviruslike element named ART-CH (avian retrotransposon from chicken genome) was obtained in the course of polymerase chain reaction-mediated cloning of avian leukosis virus long terminal repeats (LTRs) from DNAs of infected chicken cells. About 50 copies of ART-CH are present in the genome of chickens of different breeds. ART-CH is not found in DNA of quails, ducks, turkeys, or several other birds tested. The ART-CH element is about 3 kb in size, including 388 bp LTRs. The major class of ART-CH-specific RNA, also 3 kb in size, is detected in various organs of chickens. An ART-CH polypurine tract, a tRNA(Trp)-binding site, regions around the TATA box and polyadenylation signal, and the beginning of the putative gag gene strongly resemble the corresponding regions of avian leukosis viruses and EAV, the two described classes of chicken retroviruses. An open reading frame capable of encoding a polypeptide with a putative transmembrane domain is located upstream of the right ART-CH LTR. This sequence, as well as the U3 and U5 regions of the ART-CH LTR, has no obvious similarities with the corresponding parts of other known vertebrate retroviruses and retrotransposons. A short sequence upstream of the right LTR of ART-CH is very similar to sequences which flank the 3' ends of the oncogenes v-src, v-myc, v-fps, and v-crk in four different recombinant avian retroviruses and which are absent from the genomes of other studied avian retroviruses. Thus, ART-CH is a new endogenous chicken provirus that may participate in the formation of recombinant oncogenic retroviruses.