5'-Methylthioadenosine (MTA), a naturally occurring nucleoside, inhibited in vitro colony formation by murine erythroid (CFU-E) and granulocyte-macrophage (CFU-GM) progenitor cells in a dose-dependent fashion with maximal inhibition at concentrations of 2 X 10(-3) M and 1 X 10(-4) M, respectively. The inhibitory effect was reversible after up to 8 h of exposure to MTA but was irreversible after 24 h. MTA also inhibited hematopoietic progenitors in vivo. In mice given daily intraperitoneal injections of MTA for 28 days, CFU-GM were maximally reduced on day 14 to 51% of control. CFU-GM returned toward control levels by day 28 despite the continued administration of MTA. Hematocrit and leukocyte count were not reduced until day 28 and then only to 90% and 70% of control, respectively. MTA reached peak plasma levels of 2.8 X 10(-5) M 5 min after a single intraperitoneal injection of 75 mg/kg and was almost completely cleared by 60 min. These findings indicate that MTA produces reversible inhibition of murine hematopoietic progenitors both in vitro and in vivo. Despite the inhibitory effect on progenitors there is little effect on peripheral blood counts, which suggests that MTA inhibits hematopoietic proliferation without affecting hematopoietic differentiation.