The quantitative method of measuring fibroblart precursors in populations of hcmatopoietic cella dcmonstrates that their concentration in blood doea not depend on the number of heart puncturw during blood collection. This meana that fibroblaat colonies, originating in monolayer culturw of blood cella, develop from circulating blood ah. From the indices of aH-thymidine labeling it is calculated that the mean generation time for fibroblasts in seven and llday cultures is 34 to 43 hours and that the proliferative pool in fibroblast colonier ia not less than 84 and 79 per cent respectively. The concentration of fibroblast precursors among nucleated cells of peripheral blood of guinea pigs ranges between 0.2 and 2 x 1 4 6 .

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