A variety of cell line characterization methods have been useful in the detection of cross-contamination and/or misidentification of cell lines. These include karyotyping, isoenzyme analysis, DNA properties, viral susceptibility, and specific antigenicity. The work of Gartler described earlier (1) shows how, in an unusual situation, analysis of a single isoenzyme, could reveal contamination and the probable source of contamination. However, if Henrietta Lacks did not express the gene for Type A G6PD or was Caucasion rather than of African descent, a single isoenzyme analysis would be uninformative. Isoenzyme analyses have proved to be very useful in the detection of contamination involving cells of different Orders or distantly separated genera. For such situations electrophoretic mobility profiles of several isoenzymes are compared with standard profiles in order to deduce whether the cells are human, mouse, rat, dog, etc. (14). In other words, the method has usefulness for cases of interspecies contamination and very little usefulness for intraspecies contamination. Also, while isoenzyme analysis can be used to identify the mammalian order from which the cell line originated, ordinarily it cannot be used to identify the individual donor. The publications of Nelson-Rees and his associates (3,4,5 ) illustrate how severa which the cell lines belongs, l approaches, some requiring great expertise, needed to be brought to bear on identification.