The elaboration of a purely organic ferromagnet requires a precise supramolecular organization, but up to now, it has proven impossible to control or even predict the way a given molecule will crystallize. We present here results showing that the Langmuir-Blodgett technique allows for such a control. Starting from a material with antiferromagnetic intermolecular interactions, we have been able to build monolayer Langmuir-Blodgett films with ferromagnetic interactions, as evidenced by electron spin resonance measurements.