In a previous study, and in the present study, we have found that the baseline plasma samples of patients with asthma contain average levels of an endogenous heparin-like material (EHM) that is significantly higher than that noted in non-allergic, non-asthmatic controls. This material appears to have properties of both heparin and heparan sulfate. Three out of six patients responding to inhalational antigen challenge displayed an acute increment in EHM concentration that coincided with a fall in FEV values. The relation of EHM concentration to provoked asthma, or to asthma in general, remains to be determined.