We probe s-wave collisions of laser-cooled Rb-85(f = 2, m(f) = -2) ato
ms with Zeeman-resolved photoassociation spectroscopy. We observe that
these collisions exhibit a magnetically tunable Feshbach resonance, a
nd determine that this resonance tunes to zero energy at a magnetic fi
eld of 163 +/- 7 G. This result indicates that the self-interaction en
ergy of an Rb-85 Bose-Einstein condensate can be magnetically tuned. W
e also demonstrate that Zeeman-resolved photoassociation spectroscopy
provides a useful new tool for the study of ultracold atomic collision
s.