Ueber die Beziehungen von chemischer Constitution. Vertheilung und pharmakologischer Wirkung: Vortrag, gehalten im Verein für Innere Medicin am 12. December 1898.

In this speech Ehrlich explained how the molecular structure of a drug can define its action and function. It represents his first attempts at modifying the molecular structure of a compound in order to tailor the in vivo physiological actions of a drug.

Digital facsimile from wellcomelibrary.org at this link. According to Ehrlich's Collected papers, this was reprinted from v., Leyden-Festschr., Berlin: Hirschwald, 1902. That may have been its first publication.

English translation as "The relations existing between chemical constitution, distrtibution and pharmacological action," in Collected papers of Paul Ehrlich (No. 86.4).

(Thanks to Juan Weiss for this reference and its interpretation.)