CD79 is a heterodimeric molecule comprising two polypeptide chains, B29 (CD79b) and mb-1 (CD79a). It is physically linked in the surface of B cells to membrane immunoglobulin, forming the B cell antigen receptor complex. Expression of the mb-1 (CD79a) chain has been studied in leukaemias and shown to be present in most B lineage acute lymphoblastic leukaemias (ALL). In contrast, little is known about the expression of B29 (CD79b) in this condition. Two monoclonal antibodies (MoAb) were used in this study by immunocytochemistry and flow cytometry: HM57, against an intracellular epitope of the mb-1(CD79a) chain, and SN8, reacting with an extracellular epitope of B29 (CD79b). Our aim was to investigate the expression of B29 (CD79b) in the various immunological subtypes of B lineage ALL and compare its cytoplasmic and membrane expression. Seventy-nine cases were studied, including 13 chronic myeloid leukaemia in B lymphoid blast crisis (CML-BC) and 66 ALL, subclassified as early B (two), common (28), pre-B (23), mature (five) and biphenotypic with B lymphoid commitment (eight). Most cases expressed mb-1 (CD79a) in the cytoplasm. B29 (CD79b) was expressed in the cytoplasm in 65% (15/23) of pre-B-ALL and in 14% (4/28) common-ALL but it was detected in the cell membrane in only three cases of mature B-ALL, being negative in all other B lineage subtypes ALL. Three of the biphenotypic leukaemias coexpressed cytoplasmic B29 (CD79b) and mu-chain. This was also seen in two cases of CML-BC, while four cases expressed only cytoplasmic B29 (CD79b) without mu-chain. Our results suggest that during B cell differentiation, B29 (CD79b) is expressed later than mb-1 (CD79a) in the cytoplasm and parallels the cytoplasmic expression of mu-chain. B29 (CD79b) is present in the membrane at a later stage compared to its cytoplasmic expression and found in mature B blasts (B-ALL) that express membrane Ig as it is in normal and leukaemic B lymphocytes.