A series of mouse-human somatic cell hybrid lines (WILF) were derived from a hybrid that was originally thought to have chromosome 19 as its only human chromosome. In situ hybridization has been used to assess the amount of human material present in the different lines. All appear to contain different numbers of human chromosome fragments. A series of X-chromosome-specific DNA sequences hybridized against DNA from the lines revealed that material from the X long arm is present in several cases. Chromosome 19-specific DNA sequences used in a similar way show that fragmentation of this this chromosome has occurred with subsequent segregation of the fragments in different lines. The localization of these markers to various regions of chromosome 19, and their relation to the fragments observed in the WILF lines, is discussed.