AIMS--(1) To assess the diagnostic relevance of peripheral blood immunocytochemistry in hairy cell leukaemia (HCL); (2) to compare the immunostaining of bone marrow biopsy specimens with bone marrow and peripheral blood cytospins; (3) to evaluate the sensitivity of the different markers used; (4) to identify the ultrastructural localisation of DBA.44 in HCL variant. METHODS--Immunoenzymatic staining procedures, immunoperoxidase and immunoalkaline phosphatase, were used with a panel of monoclonal antibodies directed to HCL associated antigens. Ultrastructural immunostaining was performed using colloidal gold conjugated antibodies. RESULTS--HCL showed strong cytoplasmic reactivity for CD22, CD25, CD103, DBA.44, kappa, or lambda light chains. Peripheral blood diagnostic hairy cells were found in all the cases with absolute counts ranging from 0.11 x 10(9)/l up to 6.4 x 10(9)/l and values increasing with the size of the spleen. A median of 36.5% of leukaemic cells was found in bone marrow aspirates and 70% in bone marrow trephine specimens. The monoclonal antibodies CD22 and DBA.44 showed the highest and the lowest percentage of positive hairy cells, respectively; this difference was statistically significant (p = 0.0025). Ultrastructural immunolabelling with DBA.44 showed a cytoplasmic membrane localisation of the antigen in one case of HCL variant. CONCLUSIONS--(1) Immunocytochemistry is a useful technique which enhances the accuracy of diagnosis in HCL; (2) peripheral blood immunocytochemistry is recommended because it highlights hairy cells in all cases; (3) CD22 appears to be the most sensitive of the markers tested; (4) ultrastructural analysis is a useful tool in selected cases of HCL variant.