The in vitro effect of short-term culture as well as the effect of retinol (ROH), retinoic acid (RA), muramyl dipeptide [( Abu']MDP), lipopolysaccharide (LPS), and gamma interferon (IFN-gamma) on the induction of the purine metabolic enzymes, adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP), and 5'nucleotidase (5NT) in human peripheral blood monocytes (HPBM) was examined. HPBM isolated by centrifugal elutriation were cultured for up to 96 h. Following an initial time lag of 24 h, mean ADA activity from seven separate experiments as measured in nmoles/10(6) cells/h increased from a baseline of 31.3 +/- 9.3 to 57.8 +/- 16.4 (P less than 0.005) at 72 h and to 72 +/- 21.5 (P less than .025) by 96 h. 5NT activity increased from a baseline of 2.2 +/- 0.9 to a maximum of 44 +/- 10.1 by 72 h and then declined to 29 +/- 18 (P less than 0.005) by 96 h, while no significant change in PNP activity was observed. HPBM incubated for 3 d with optimal concentrations of LPS, RA, and IFN-gamma had increases in ADA and 5NT activity ranging from three- to 10-fold compared to HPBM cultured in media alone, whereas no effect was observed with ROH and [Abu']MDP. RA, but not ROH, significantly enhanced ADA activity in a monocytic leukemia cell (THP-1) line. Addition of RA or the tumor promoter, phorbol 12-myristic 13-acetate (PMA), to HPBM or THP-1 cells resulted in significant increases in 5NT activity with opposite effects on ADA activity. These findings suggest that the biological mechanisms associated with differentiation in normal and malignant monocytes seem to be related and that the sequence and degree to which the various differentiation agents induce the enzyme elevations are also related to the mechanisms of activation/differentiation.