Blood mononuclear cells from ten healthy donors and cells from fifty cultured human lymphoblastoid lines were tested for the capacity to form rosettes with sheep erythrocytes. The effect of adding glutaralhehyde before scoring rosettes and the effect of substituting glutaraldehyde-fixed for fresh sheep cells were both examined. The results suggest that glutaraldehyde increases the apparent proportion of rosette-forming cells by altering the sheep erythrocytes in such a way that they will adhere to some B lymphocytes. Although a small number of cells carry both surface immunoglobulins and receptors for fresh sheep erythrocytes, the rosette test with fresh sheep cells identifies T lymphocytes more accurately in the absence of glutaraldehyde than in its presence.