Abstract: A method for separating plasma in a minute amount of a blood sample diluted with a blood dilution buffer is provided. When a blood dilution buffer is added to a minute amount of blood (about 20 to 100 ?L) so as to assay the biological blood components in the resulting diluted plasma, it is necessary to separately store and transfer the diluted blood cell component and the diluted plasma component. The present inventors found that it is possible to separate diluted plasma from diluted blood cells using a polymer gelling agent for plasma separation by means of putting a polymer gel for plasma separation in the container with a blood dilution buffer. In such case, diluted blood cells are transferred to the lower portion of the polymer gelling agent and dilute plasma is transferred to the upper portion of the same. Assay of the biological blood components using a minute amount of blood can be utilized for early detection of health status, diseases, and presymptomatic states.