Using Northern-blot analysis we have studied theexpression of T-cell receptor (TCR) a, b and y chain genes inprimary cells obtained from 36 leukaemic patients. Fifteenpatients had myeloid leukaemias, two had T-cell leukaemias,and 19 leukaemias corresponding to various stages of Blymphocyte differentiation. We observed that truncated TCRbeta mRNAs were produced in B-cells at relatively high levelseven in the absence of detectable gene rearrangements. Ti alfa mRNAs of abnormal size were also frequently found. Suchtranscripts were not detectable in total RNA extracted fromleukaemic myeloid cells. Factors allowing Ti alfa and beta genetranscription must be active in leukaemic pre-B and B cellsbut not in myeloid cells. Neither B-lineage nor myeloidleukaemias expressed TCR y gene at detectable levels.