Studies of human peripheral lymphocytes (HPL) from healthy subjects and of human lymphoblastoid cell lines (HLC) revealed spontaneous formation of rosettes with Macaca speciosa monkey red blood cells (MRBC). The percentage of HPL forming rosettes with MRBC paralleled the percentage of HPL exhibiting markers ascribed to human B cells. The MRBC-rosetting cells were positive for membrane-bound immunoglobulin (MBIg). Furthermore, by employing a mixed rosette method, lymphocytes carrying both EAC3dmo and FITC-MRBC were encountered. In contrast, no lymphocytes carrying both SRBC and FITC-MRBC were observed. Ninety-six per cent of purified B cells obtained at the interface after interaction of HPL with SRBC and Ficoll-Isopaque centrifugation formed rosettes with MRBC. In contrast, only 8% of an enriched population of T cells obtained at the interface after interaction with EAC3dmo and Ficoll-Isopaque centrifugation formed MRBC-rosettes. Incubation of HPL with MRBC and subsequent centrifugation on Ficoll-Isopaque resulted in a highly enriched T cell population at the interface. Several lymphoblastoid cell lines with B cell characteristics, but not two cell lines with T cell characteristics, formed spontaneous rosettes with MRBC. Inhibition experiments showed that the structure on the lymphoid cell surfaces mediating the adherence of MRBC was independent of the MBIg as well as the recptors for IgGFc, C3b, and C3d. It appears that rosetting with MRBC will provide a new detection marker for human cells classified as B type and a technique for obtaining an enriched T cell population.