House dust mite (Dermatophagoides farinae) has been shown to activate the complement system in vitro by both the classical and alternative pathways in normal serum and in two groups of (skin test positive and skin test negative) patients with bronchial asthma. The activation was dose dependent in the lower range of allergen concentrations. Skin test positive asthmatic subjects showed the maximum activation. Skin test negative patients had activation between skin test positive patients and normal subjects. However, at the higher allergen concentration normal serum and bronchial asthma patients showed similar results.