The antigen is a 154-180 kDa protein as determined by immunoprecipitation. The epitope has not been further characterized. Antigen distribution: Isolated cells: Granulocytes, monocytes, positive on blood monocyte-derived macrophages days 0-28. Dull staining with 95 % cord blood CD34+ cells, weak reaction on resting neutrophils. Staining increases with maturation of granulocytes. Treatment with fMLP, TNF, GM-CSF and temperature causes positive shift with granulocytes and monocytes. PMA treatment of granulocytes leads to a decrease in the number of positive cells. Positive with cell lines U937, DH39 & MEP, HL-60, NB4, HMC-1. Tissue sections: X-6 is positive on lining cells in different infiltrations and endothelial cells in the synovial membrane of rheumatoid arthritis patients. The staining pattern is similar to CD34 in these patients. Specificity: Human: granulocytes, monocytes, platelets. Other species: not tested.