The number and morphology of basophil leukocytes in simultaneously taken blood and bone marrow specimens have been compared. Healthy subjects and patients with various diseases but with apparently normal bone marrow have been used. Blood contained more degranulated basophils than bone marrow. The degranulation is both due to a shift from darkly stained to faint granules and to a shift from cells with numerous intracellular granules to those with extracellular spray or few residual granules. The mean percentage of basophils in blood was 0.47 [plus or minus] 0.05. In bone marrow 0.83 [plus or minus] 0.03% of all nucleated cells counted were basophils. No correlation was found between the percentage of basophils in blood and bone marrow. Mast cells were found in 38% of the bone marrow specimens examined. There was no correlation between the amount of mast cells (0.02-0.001%) and basophils.