As a preliminary approach to developing a biochemical assay for detecting humoral regulators of granulocyte maturation in the normal and inflammatory states, studies were carried out on the effects of normal and inflammatory sera on the incorporation of 14C-glucosamine into the glycoproteins of bone marrow granulocytes in vitro. We observed that, relative to normal serum, inflammatory serum had a marked stimulatory effect on 14C-glucosamine incorporation into these glycoproteins. This property of inflammatory serum reached a maximum at about 8 h after the initiation of inflammation in vivo and preceded the maximum increase in the mitotic activity of granulocyte precursors in the marrow by 18 h. It was also found that normal serum contains both dialyzable and heat-sensitive nondialyzable factors that inhibit 14C-glucosamine incorporation into bone marrow granulocytes in vitro. Data are presented which indicate that the stimulatory effect of inflammatory serum is most likely due to a nondialyzable factor which is capable of blocking the effect of the inhibitors present in normal serum.