Type A chrysotile fibres (white asbestos) were tested in vitro for activation of the complement system. Fibres were incubated in normal human serum (NHS), factor B-depleted human serum, and normal and C4-deficient guinea-pig sera; the supernates were assayed for the remaining complement activity. Activation of the alternative pathway (AP) was shown in three ways. First, quantitative measurement of factor B; second, kinetic analysis of rabbit red blood cell lysis in whole alternative pathway (AP) and factor B lytic assays; third, qualitative measurement of C3 and factor B conversion by crossed immunoelectrophoresis. No C3 convertase activity could be demonstrated on the fibres but other possible mechanisms of AP activation are discussed. Magnesium itself is not responsible for this activation because acid-treated fibres retain this property. The early classical pathway is not involved as shown by normal whole complement activity of a factor B-depleted human serum and the absence of decrease of C4 functional activity. Knowing that complement proteins are present in pulmonary alveoli, mainly provided by cell synthesis, we suggest that complement activation in vivo may be relevant to the genesis of the chronic inflammation and fibrosis in the lung.