Cells from peritoneal exudates of rabbits sacrificed 3 days after an intraperitoneal injection of sterile mineral oil were grown in tissue cultures in medium 199 (75 per cent); normal rabbit serum (25 per cent). Antibody produced by the cells was assayed by an hemagglutination technique in which the antigens used were adsorbed to formalinized tanned sheep erythrocytes. These sensitized cells agglutinate in the presence of antibody specific to the adsorbed antigen. It has been demonstrated that:

Peritoneal exudate cells produced hemagglutinating antibody to bovine gamma globulin (BGG) in a replicating tissue culture system for approximately 3 weeks when taken from animals given either primary or secondary injections of BGG. The mean hemagglutinating titer was 30 for the primary and 32 for the secondary systems. Since the other cell types did not persist, it is felt that monocytes were responsible for these results.

Monocytes taken from normal rabbits and exposed to either BGG or egg albumen (EA) in vitro produced titers of 28 for about 2 weeks.

Monocytes taken from rabbits given hyperimmunizing injections of BGG produced titers of 147 for about 1 week.

Endotoxin from Salmonella typhosa caused the monocytes to form antibody as if they had been taken from hyperimmunized rabbits. This was true both when the antigen was given in vivo together with the endotoxin as well as when the cells were exposed to antigen in vitro. The titers were 223 and 97, respectively.

Neither freshly harvested nor cultured monocytes were phagocytic for carbon particles or bacteria in vitro. Monocytes in tissue culture appeared to assume the morphology of fibroblasts, but did not stain with the characteristics of fibroblasts. The morphologic changes and staining characteristics of monocytes in tissue culture have been described.

The implications of these findings have been discussed and an attempt made to integrate them into general biological theory.

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