Astragalus membranceus (Fish) Bunge Var. mongholicus (Bge) Hsiao is a Chinese herb considered as an effective traditional anti-ageing material. The two isomers of 4-hydroxy-5-hydroxymethyl-[1,3]dioxolan-2,6'-spirane-5',6',7',8'-tetrahydro-indolizine-3'-carbaldehyde (HDTIC), HDTIC-1 and HDTIC-2, were extracted from the herb. We chose them to investigate their effects on replicative senescence in vitro. In this study, we observed the effects of HDTIC-1 and HDTIC-2 on morphology, replicative lifespan, and specific markers related to replicative senescence in human fetal lung diploid fibroblast (2BS cell). Results have shown that both the HDTIC-1 and HDTIC-2 maintain non-senescent phenotype of 2BS cells even at late population doubling (PD) and increase cumulative population doublings (CPDs) by at least 15-20PDs. The senescence-associated-galactosidase (SA-beta-gal) positive cell rates of late PD cells grown from early PD in medium containing HDTIC, were much lower than that of late PD control cells, and similar to that of young cells. HDTIC also improved cell growth and proliferation and promoted the entry of 2BS cells from G0 or G1 phase to S-phase. In addition, the advanced glycation end product (AGE) levels of late PD cells grown from early PD in DMEM containing HDTIC decreased significantly compared with those of late PD control cells. Taken together, the results strongly suggest that both the HDTIC-1 and HDTIC-2 delay replicative senescence of 2BS cells, and indicate that the senescence-delaying effect of HDTIC appears to be due to its many biological properties including its potentials of proliferation improvement, inhibitory effect of AGE formation, and its antioxidant activity. The differences of optimum concentrations of HDTIC-1 (0.1 microM) and HDTIC-2 (1.0 microM) for delaying senescence also indicate that the structure of HDTIC may be very sensitive to its activity.