Rat colony-forming unit spleen is OX7 positive, W3/13 positive, OX1 positive, and OX22 negative.

Using the monoclonal antibodies OX7 HL, W3/13 HLK, OX1 HLK, OX22, and the technique of fluorescence activated cell sorting, it was possible to characterize the phenotype of the rat marrow CFU-S as OX7 upper 20% positive, W3/13 lower 50% positive, OX1 positive, and OX22 negative. OX7 recognizes an antigenic determinant expressed on the Thy1 glycoprotein, W3/13 recognizes a determinant expressed on some sialoglycoproteins, OX1 recognizes all four apparent molecular weight forms of leukocyte common antigen, while OX22 recognizes only the high molecular weight forms of leukocyte common antigen. It was determined that the concentration of OX7 upper 20% positive, W3/13 lower 50% positive, OX1 positive, and OX22 negative cells in the marrow was 3085 +/- 1446/10(6) cells. For comparison, the calculated concentration of marrow stem cells using a 2-h seeding efficiency was found to be 501 CFU-S/10(6) cells and, using a 24-h seeding efficiency, it was found to be 1415 CFU-S/10(6) cells. Although requiring further refinements, these results suggest that an assessment of CFU-S marrow concentration might be achieved using multiparameter flow cytometry. Also, a technique for the conjugation of the Fab' fragment of the monoclonal antibody OX7 to phycoerythrin is described.