Ki-M9R detects a 49 kDa cytoplasmic antigen. The epitope has not been further characterized. Antigen distribution: Isolated cells: Ki-M9R is negative with all peripheral blood cells and cells of the bone marrow or peritoneal cavity. Tissue sections: Ki-M9R specifically recognizes antigen trapping macrophages of the B-cell system such as sinus lining cells and dendritic reticulum cells. It is also positive on splenic metallophils. Macrophages in T-cell areas are negative with Ki- M9R. None of the macrophages in non-lymphoid organs are detected by Ki-M 9R. No cross-reactivity was found with other cell types at different tissue sites except with hepatic sinusoids. In vivo adminstration of Ki-M9R reveals a direct cytotoxicity to, and the elimation of Ki-M9R positive cells. Animals survive this treatment. Specificity: Rat: subpopulation of macrophages. Other species: not tested Comments: During the course of an infection the number of antigen trapping cells decreases while the number of antigen presenting cells increases. Antigen presenting Ki-M4R Ki-M9R Antigen trapping Course of an infection Unpublished data raised in The Institute of Pathology, Michaelisstrasse 11, 2300 Kiel, Germany