Utilizing a recently reported characteristic of monocytes to aggregate in cold, a new procedure for obtaining monocyte-rich and monocyte-depleted mononuclear fractions from human blood is described. After aggregation in cold and adherence to plastic, a fraction containing 88% monocyte (LeuM3 CD14+) is obtained. After treating the supernatant, which separates from the aggregates, with a monocyte lysosomotropic agent (L-Leucine Methyl Ester), a fraction containing 0% monocyte (LeuM3 CD14+) and 88% lymphocytes (Leu4 CD3+ and Leu 12 CD19+) is obtained. In round-bottom culture wells, this monocyte-depleted fraction produced immunoglobulins in response to pokeweed mitogen (5,826 +/- 2,356 IgM micrograms/ml), to a degree not significantly different that that produced in the presence of monocytes (7,426 +/- 3,347 IgM micrograms/ml). This suggests the presence of cells in the lymphocyte fraction, that are not monocytes but are capable of antigen presenting.