A probe derived from the 3' end of the CML breakpoint cluster region (bcr) was localized in chronic myelocytic leukemia (CML) cases with complex Philadelphia translocations, [t(8;9;22)(q13;q34;q11) and t(12;9;22)(p11;q34;q11)], and with "masked" Ph chromosomes, [t(9;5;22)(q34;q31;q11) and t(9;22)(q22;q34)], by a chromosomal in situ hybridization technique. In some cases, the 3' bcr rearrangements in the DNA were examined with Southern blot analysis. In each case, a significant accumulation of grains hybridized to the 3' bcr probe was observed at chromosomal segments derived from the long arm of a chromosome #22. In some cases, the accumulation of grains was detected on both translocated segments derived from the 22q and terminal portions of Ph chromosomes; Southern blot analysis revealed that breakage in chromosome #22 occurred within DNA sequences of the 3' bcr probe involved in the Ph translocations. In a CML cell line, K562, no accumulation of grains hybridized to the 3' bcr probe was detected, except at 22q11 of the normal chromosomes #22; Southern blot analysis of this cell line revealed that the 3' bcr sequences were missing. Thus, the data presented here suggest a complexity in the formation of "masked" Ph chromosomes.