51Cr-labeled thymus cells exposed in vitro to 1:1000 dilution of rabbit anti-mouse lymphocyte serum (RAMLS) and injected into mice localized in the livers of recipient mice to a greater extent than cells exposed to normal rabbit serum (NRS). The increased liver uptake was due to phagocytosis of a proportion of the injected cells. The percentage of thymus cells opsonized by RAMLS could be calculated from a knowledge of the relative liver-to-spleen localization ratios in mice injected with cells preincubated in NRS and in RAMLS. The percentage is given by The percentage of cells opsonized by different preparations of RAMLS varied widely (5% to 90%). There was a good correlation between the ability to opsonize thymus cells and to depress the immune response to sheep erythrocytes.