A flow cytometric method for the quantitation of reticulocytes was refined for routine laboratory use. Blood (2 microliters) is added to 2 ml of 0.4 microM thiazole orange in phosphate buffered saline, incubated at room temperature for 90 minutes, and analysed on a Coulter EPICS Profile flow cytometer, with gating for red cells on the basis of forward and right angled light scatter. Blood (2 microliters) is also incubated with phosphate buffered saline alone as an unstained control. The adult reference range (mean +/- 2 SD), established from 30 laboratory personnel, is 19.4-59.2 x 10(9)/l (0.2-1.6%). Comparison of this technique was made on 39 selected patient samples with visual counting of cells stained with brilliant cresyl blue. The correlation between the two methods was 0.99 with slope 0.96 and intercept 0.02. The precision of the automated technique in three subjects with reticulocyte counts of 0.12%, 1.84%, and 14.3% was 33.3%, 7.3%, and 1.4%, respectively (coefficient of variations). In three patients studied serially after intensive chemotherapy, in whom the reticulocyte count quantitated by routine visual methods approached zero (0-0.1%) for eight to 18 days, the automated counts varied between 0 and 0.5%. Flow cytometric reticulocyte counting is thus a simple and highly reliable methodology for the quantitation of normal and raised reticulocyte counts but cannot be reliably used to quantitate a subnormal level.