Human leukemia-lymphoma (LL) cell lines represent important tools for experimental research. Among the various problems associated with cell lines, the two most common concern contaminations: (1) cross-contamination with unrelated cells and (2) contamination with microorganisms, in particular mycoplasma. The bad news is that about one-third of the cell lines are either cross-contaminated or mycoplasma-infected or both. The good news is that there are means to recognize and overcome these problems. In cases where, during attempts to establish new LL cell lines, primary LL cultures are cross-contaminated with continuous cell lines, intended new cell lines simply cannot be established ("early" cross-contamination). In cases of "late" cross-contamination of existing LL cell lines where the intrusive cells have a growth advantage, the original ("uncontaminated") cell lines may still be available elsewhere. DNA fingerprinting and cytogenetic analysis appear to be the most suitable approaches to detect cross-contaminations and to authenticate LL cell lines. A different but related aspect of "false" LL cell lines is the frequent misclassification of cell lines whereby the actual cell type of the cell line does not correspond to the purported model character of the cell line. Mycoplasma infection can have a multitude of effects on the eukaryotic cells which, due to the variety of infecting mycoplasma species and many other contributing parameters, cannot be predicted, rendering resulting data questionable at best. Practical procedures for the detection and elimination of mycoplasma contamination have been developed. Diagnostic and preventive strategies in order to hem the alarming increase in "false" and mycoplasma-positive LL cell lines are recommended.