The activities of acid and neutral maltase were measured in lymphocytes, granulocytes, and platelets isolated from controls and from 5 patients with late-onset acid maltase deficiency (AMD). Lymphocytes from patients had markedly decreased activity of acid maltase and elevated neutral/acid ratios. In granulocytes, acid maltase was also lower than in controls, but significant activity was retained at pH 4: neutral/acid ratios were consistently elevated. Normal platelets had low acid and high neutral maltase activities: both enzyme activities varied within wide ranges and patients could not be distinguished from controls. The variable proportion of different cell types in unfractionated leukocyte preparations may yield unreliable values when used for detection of AMD. However, lymphocytes isolated from 20 ml of blood provide a readily accessible and reliable source of tissue for accurate diagnosis.