We have recently reported a new and rapid assay to measure plasma holotranscobalamin II (holo TC II) as a means of exploring vitamin B12 status. In order to further evaluate the significance of plasma holoTC II in determining tissue cobalamin, we have chosen the red blood cell-vitamin B12 (RBC-B12) assay as a measure of tissue vitamin B12 content and studied the relationship between RBC-B12 and plasma holoTC II levels. Plasma holoTC II and RBC-B12 concentrations were concomitantly assayed in 20 hematologically normal controls and cancer patients. In our groups of controls, the mean value of RBC-B12 was determined as 241 +/- 51 pg/ml of packed erythrocytes (PE) with a range varying from 180 to 355 pg/ml PE. Preliminary results obtained in 32 cancer patients revealed lower holoTC II and RBC-B12 levels than the control group and a required threshold value of 70 pg/ml of holoTC II in order to maintain a normal RBC-B12 greater than 180 pg/ml PE.