Abstract 1. 1. A series of 800 comparative packed cell volume determinations made by the standard centrifuge tube method on fifty 100 c.c. samples of dog's blood demonstrates that a 1.5 per cent solution of sodium oxalate gives values practically identical with those obtained with heparin. 2. 2. The minimum error found for the standard tube method was approximately 1 per cent, obtained when 10 c.c. quantities of blood were employed. The error is proportionately greater when smaller quantities of blood are used. It is slightly less with 12 c.c. than with 15 c.c. tubes. 3. 3. Packing was complete after one hour of rotation in 97.5 per cent of cases.