Anaphylactic tests in guinea pigs and double-diffusion tests with rabbit serum produced no evidence of antigen-like properties of polyvinylpyridine-N-oxide (PVNO). It is very unlikely therefore that the occasional clinical observations of collapse-like states, occurring when PVNO injection is resumed after a treatment-free interval are of allergic nature. These are most probably due to erythrocyte aggregation following excessively rapid injection of PVNO, producing collapse in the terminal pulmonary vasculature by a reflex action. This reaction can be avoided by slow intravenous injection or infusion.