The quantitative method of measuring fibroblast precursors in populations of hematopoietic cells demonstrates that their concentration in blood does not depend on the number of heart punctures during blood collection. This means that fibroblast colonies, originating in monolayer cultures of blood cells, develop from circulating blood cells. From the indices of 3H-thymidine labeling it is calculated that the mean generation time for fibroblasts in seven and 11-day cultures is 34 to 43 hours and that the proliferative pool in fibroblast colonies is 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 × 10-5.