The levels and activity of tissue transglutaminase were studied in human peripheral blood monocytes during differentiation into macrophages in vitro. The enzyme was present at low levels in freshly isolated monocytes (less than 20 ng/mg cell protein) but increased 50-fold during 10 d of adherent culture in autologous serum, reaching levels of 0.1% of total cellular protein. The rate of appearance of tissue transglutaminase in monocytes was accelerated by low levels of lipopolysaccharide. The half-life of disappearance of transglutaminase from human monocytes was 11 and 7 h in 2-d-old and 10-d-old cells, respectively. Treatment of 1-day-old monocytes with actinomycin D for 24 h blocked the increase in transglutaminase levels. These results indicated that the induction of gene transcription and protein synthesis was responsible for the increased transglutaminase levels and activity observed with cultured human monocytes. The induction of tissue transglutaminase may be a component in the in vivo differentiation of human monocytes into macrophages.