Normal mouse sera inhibited auto-erythrocyte rosette formation by mouse thymus cells. Serum inhibitory factor was associated with high density lipoprotein (HDL) isolated from mouse sera with sequential flotation ultra-centrifugation techniques. Other portions of normal mouse sera i.e., very low density lipoprotein (VLDL), low density lipoprotein (LDL), lipoprotein-free serum, and delipidated HDL had little or no inhibitory activity. A 20-fold purification with 68% recovery of total serum activity was obtained by this isolation procedure. HDL from the serum of BALB/c, C57BL/6, C3H, and NZB/BIN mice inhibited BALB/c auto-rosettes. Inhibition was observed when lymphocytes were preincubated with HDL and then washed before assaying, but there was no inhibition when erythrocytes were pretreated.