The elastic properties of lambda-digest DNA adsorbed on a mica substrate were investigated using an atomic force microscope (AFM) in fluid. The tip of the AFM probe was pushed into DNA molecules and pulled back repeatedly at various points on the sample while force versus distance curves were measured. We observed that two types of overstretching transitions from B-form DNA to S-form DNA in the measured force-extension curves. The force corresponding to the B - S transition in the first was 67 +/- 7 pN and in the second was 116 +/- 6 pN, respectively. We calculated the stretch modulus of DNA using the force-extension curves accompanied by the B-S transition. The results obtained from the two different force-extension curves were consistent with each other and were also in accord with the low salt data in the literature. (C) 2004 Elsevier B.V. All rights reserved.