The DNA sequence requirements for mammalian centromere function have been investigated by re-introducing human YAC clones containing either centromeric or non-centromeric sequences into hamster and human cells. All YACs integrated into the host chromosomes. In most cell lines produced by spheroplast fusion into hamster cells, intact copies of the YAC and a large amount of yeast DNA were found. Cell lines produced by lipofection into human cells usually contained simple structures without yeast DNA. YACs containing Y alphoid DNA reformed several of the properties of a centromere, including a cytogenetically visible constriction, CREST antiserum binding and disruption of anaphase chromosome movement. In contrast, YACs containing non-centromeric sequences produced none of these results. This work suggests that a few hundred kb of alphoid DNA is sufficient to reconstitute several important features of a centromere.