Hairy cell leukemia is a chronic B cell leukemia. The presence of surface Ig (SIg) of gamma or multiple isotypes on the cells locates HCL at a rather mature stage of B cell differentiation. The reactivity of HC with McAb is in accordance with this concept (T65-, OKM1+, FMC7+, BA-1-). To relate HCL to other chronic B cell leukemias, a morphologic classification was developed that distinguishes, besides HCL, five subtypes of chronic B cell leukemia. CLL showed weak staining for SIg of mu +/- delta class. The phenotype with McAb was T65+, OKM1-, FMC7-, BA-1+. Prolymphocytic transformation of CLL had essentially the same membrane phenotype. LPL often had brighter SIg of mu +/- delta class with gamma or multiple isotypes in about half of the cases. McAb gave a T65-or+, OKM1-or+, FMC7-or+, BA-1+ phenotype. The same surface-marker profile was found in CL. Finally, PLL showed bright SIg of gamma or multiple isotypes in the majority of cases and reactivity with McAb according to a T65-or+, OKM1+, FMC7+, BA-1+ pattern. The various immunologic phenotypes of the morphologic subtypes showed a considerable overlap. The various chronic B cell leukemias should be located in the scheme of B cell differentiation in the sequence CLL-LPL/CL-PLL-HCL.