Between December, 1984, and May, 1986, 98 CSF samples were sent to a central laboratory by postal express. The samples could be kept in a medium for up to 24 hours after the lumbar puncture. The quality of the cells proved to be good. Excluded were 5 samples delayed in delivery and 13 samples contaminated with blood, defined as the macroscopical presence of blood. The microscopical presence of erythrocytes in the cytocentrifuge preparation can make interpretation of the results difficult. Especially when leukemic blasts are present in the blood, extreme caution is necessary. A total of 71 samples could be studied by cytomorphology as well as by TdT-IF. When cytomorphological leukemic blasts were present, this was confirmed by TdT-IF positivity in all cases. But in 6 of 71 samples, TdT-IF was positive without the presence of cytomorphological leukemic blasts. Follow-up of these patients will show whether the therapeutic regimen has to be changed.