Chromosomal translocations involving the human 12p13 band frequently affect the TEL gene, usually resulting in gene fusion between TEL and genes encoding proteins of various types. The most frequent 12p13 translocation is the t(12;21)(p13;q22), which recombines TEL with the AML1 gene on chromosome 21 and is frequently associated with deletion of the untranslocated TEL allele. Using antisera against different parts of TEL and against the AML1 proteins, we undertook Western blot and immunofluorescence analyses of leukemic samples with and without 12p13 abnormalities. In t(12;21) samples, TEL-AML1 was detected as several protein species in the nuclei, whereas the AML1-TEL protein, was inconsistently expressed. AML1 was found to be expressed but no normal TEL proteins were detected. A survey of the TEL proteins in a panel of human leukemic samples without t(12;21) revealed a variation in the ratio of TEL protein isoforms. We also analysed a leukemic cell line bearing a t(12;22)(p13;q11) that was found to affect the 5' untranslated (UT) region of TEL and to be associated with inactivation of the untranslocated TEL allele. No MN1-TEL fusion could be detected upon RT-PCR analysis, in contrast to the previously investigated t(12;22). Strikingly, extremely low levels of apparently normal TEL proteins, expressed from the translocated allele, were detected by Western blot analysis. These results suggest that the level of TEL expression can be important for leukemogenesis.