Milkweed latex collected in mid-July was coagulated in acetone, the coagulum purified, and the rubber examined by chemical and X-ray diffraction methods. Chromic acid oxidation analyses indicate that the monomer is isoprene. The X-ray evidence verifies these results and, further, fixes the polymer as the cis-configuration (rubber) as opposed to the trans-configuration (gutta-percha) of polyisoprene.The solvent-extracted rubber from the leaves appears from the chromic acid oxidation results to be isoprene, but so far no polyisoprene diffraction diagram has been obtained. It is therefore likely that the solvent-extracted leaf rubber is an isoprene polymer of low molecular weight.