Chromosomal abnormalities involving 3q27 have recently been associated with diffuse large B-cell lymphomas and, less frequently, with follicular lymphomas. Molecular studies have led to the identification of the BCL-6/LAZ-3 gene, located at 3q27 and coding for a putative zinc-finger protein that might act as a transcriptional regulator during cell differentiation and development. Rearrangement of BCL-6 results in truncation of the gene in its 5' portion, leaving the protein intact; a resultant deregulation of its expression has been hypothesized. In order to test this hypothesis, the expression of BCL-6 protein was investigated in human reactive lymphoid tissue and compared with a group of non-Hodgkin's lymphomas (NHLs) with or without 3q27 anomalies and/or BCL-6 gene rearrangement. BCL-6 protein is consistently expressed in reactive lymphoid tissues, where it is restricted to the follicle centre. The protein is also widely expressed in NHL: all follicular lymphomas tested showed a pattern of expression similar to the reactive B follicle, independently of the presence of BCL-6 gene rearrangement and/or 3q27 anomalies. In the diffuse large B-cell lymphomas, there was more variation in BCL-6 expression, but a correlation with 3q27 anomalies and/or BCL-6 rearrangement was not found. Deregulation of the BCL-6 gene did not result in an aberrant tissue expression as detected by immunohistochemistry.