Auer rod-like inclusions (ARLI) can be seen in B cell malignancies but are very rare in prolymphocytic leukemia (PLL). Prolymphocytes with ARLI from a patient with PLL were examined using cytochemical and immunochemical techniques and transmission electronmicroscopy (EM). The lymphoblastic cells gave a positive reaction with antibodies to the gamma heavy chains and kappa light chain. On EM the ARLI were 1–2 μ m long, consisting of election-dense crystalline matrix, with a periodicity of 60 Å and contained within a single unit membrane. The production of IgG by PLL cells with ARLI has not been previously reported. It is emphasized that in PLL the lymphoblasts can have inclusions resembling Auer rods. Cytochemical and immunochemical studies should be done to avoid a misdiagnosis of acute nonlymphoblastic leukemia.