The invention relates to novel crystalline forms of 3-[[3,5-dibromo-4-[4-hydroxy-3-(1-methylethyl)-phenoxy]-phenyl]-amino]-3-- oxopropanoic acid, said crystalline forms being characterised by a powder X-ray diffraction pattern having major peaks at either 2.theta.=16.1.+-.0.2, 20.1.+-.0.2, 20.7.+-.0.2, and 24.2+0.2; or 2.theta.=9.0.+-.0.2, 14.7.+-.0.2, 19.6.+-.0.2, 21.6.+-.0.2, and 24.3+0.2.