This preprint shows that de novo designed single-domain antibodies (VHHs) can neutralise α-cobratoxin, the long-chain neurotoxin of the monocled cobra, and protect mice against a lethal dose. Flow Induced Dispersion Analysis (FIDA), on a Fida 1 instrument, was the method of first choice for finding which designs bound: the whole set of purified candidates was screened in free solution against fluorescently labelled α-cobratoxin, with complex formation read directly as an increase in the hydrodynamic radius of the labelled toxin. Only the target carried a label, the designs themselves needed none and nothing was immobilised on a surface, and sub-stoichiometric binder was enough to see the complex, so a whole designed library could be triaged in solution from small amounts of protein. Only the candidates FIDA identified were taken forward to a surface-based method for the follow-up characterisation. It is an example of using FIDA as the first-choice, in-solution binding screen across a designed protein library, selecting the few candidates worth characterising further.