Continuous flow centrifugation leukopheresis lasting 3 to 4 h was performed 18 times on 16 normal donors. The total yield of leucocytes collected was 1.59 +/- 0.14 x 10(10) (mean +/- SE). Comparison of leucocyte yields on the first, second, third and fourth hour of leukopheresis indicated a progressive increase with time in 15 out of 18 procedures, with 3.6 times more leucocytes collected within the last hour of leukophoresis as compared to the first hour. This increase was independent of the volume of leucocytes collected per unit time, since an equally significant increase was found when calculated per 100 ml of leucocyte concentrate. Although the mechanism of this improvement in yield with time is unknown, the present data indicate, that moderate prolongation of the duration of leukopheresis may result in an increase in efficiency which is comparable to that produced by the use of hydroxyethyl starch, etiocholanolone or corticosteroids.