Biopsies from 319 haematopoietic neoplasms were immunostained for intracellular leucocyte-specific protein 1 (LSP1) to assess its distribution and to compare its diagnostic value with that of CD45 (leucocyte common antigen: LCA). Most small B-cell neoplasms expressed LSP1, but one third of diffuse large B-cell lymphomas (DLBCL) were LSP1 negative. Among the cases with DLBCL (76 samples) tested for both LSP1 and CD45, one fifth expressed only CD45, but five samples were LSP1-positive and negative for CD45. The latter pattern was also seen in four of nine myelomas. Five out of 14 T-lymphoblastic lymphomas co-expressed LSP1 and CD45, and three cases were LSP1 negative and CD45-positive. Most peripheral T-cell lymphomas co-expressed LSP1 and CD45. All anaplastic lymphoma kinase (ALK)-negative lymphomas of anaplastic large cell morphology (T and null phenotype) expressed LSP1 although the percentage of LSP1-positive tumour cells was variable, however, only seven out of 30 cases with ALK-positive lymphoma were LSP1 positive. LSP1 was expressed on Reed-Sternberg cells in 60 out of 66 cases with classic Hodgkin's disease but neoplastic cells were usually negative in lymphocyte predominant Hodgkin's. This study confirms the wide expression of LSP1 within haematopoietic neoplasms and its diagnostic value for a minority of lymphoid tumours that have lost CD45 expression. Furthermore, the strong expression of LSP1 in classic Hodgkin's disease, contrasting with its heterogeneous expression in ALK-negative anaplastic lymphomas, may help to distinguish the latter lymphomas from patients with tumour cell-rich Hodgkin's disease.