The present invention relates to agonists of the S1P4 receptor, which are selective for the S1P4 receptor over one or more of the S1P1, S1P2, S1P3 or S1P5 receptors of at least 10 fold, in particular new indol-alanine derivatives of structure I, process for their production, their uses, in particular in transplantation, and pharmaceutical compositions containing them wherein R1 is phenyl or naphthyl, wherein phenyl is substituted by one or two of halogen, C1-6alkyl, C1-6alkoxy or phenylC1-6alkyl; and R2 is hydrogen or C1-6alkyl; in free or salt form.