We evaluated the prototype of a "Microtainer" (800-mul) tube (Becton-Dickinson Co., Rutherford, N.J. 07070), In which a semi-solid silicone material is used to separate serum from erythrocytes on centrifugation. We compared results for 18 serum analytes with those obtained for specimens collected by the "dripping" method of capillary blood collection. The serum was examined microscopically for platelets and leukocytes from both normal and leukemic specimens. The cellular elements were all contained in the erythrocyte layer. Hemolysis was negligible. Means for paired samples did not differ significantly, except for Na+, for which the difference was not clinically significant. Values for potassium and lactate dehydrogenase were unchanged 24 h after collection, even though the serum was left in the collecting device.