MDL ISIS/Draw 2.5 was a foundational 2D chemical structure drawing software that became a standard in academic and industrial chemistry laboratories. Developed by MDL Information Systems, it allowed scientists to sketch complex molecules, chemical reactions, and biological entities with "chemical intelligence"—meaning it understood valence limits and bond angles.
While the software is often searched for as "freeware," MDL (now part of Dassault Systèmes) traditionally licensed this software. mdl isis draw 2.5 free download