SUMMARYIt is not possible to distinguish between direct pharmacological effects and immune-mediated hypersensitivity reactions by clinical observation alone and errors may occur in the absence of laboratory tests. A convenient and simple test is the measurement of plasma complement C3 consumption and conversion in sequential blood samples taken at intervals over the 24 h following an adverse response.Interest in adverse reactions to i.v. anaesthetic agents, particularly those appearing to be related to hypersensitivity, has increased in recent years (Clarke et al., 1975). Reports of reactions to propanidid (Epontol) attributed to hypersensitivity are frequent (Clarke, 1974). Reports of hypersensitivity to thiopentone (Dundee and Wyant, 1974) and methohexitone (Driggs and O'Day, 1972), although fewer than those associated with propanidid, are similar in character. The new steroid anaesthetic, Althesin, has caused a significant number of adverse reactions of the hypersensitivity type (Sutton, Garrett and McArdle, 1974). Unfortunately it is not possible to distinguish readily between direct pharmacological effects and immunemediated hypersensitivity reactions, the latter implying previous sensitization of the patient either to the anaesthetic agent itself or to a cross-reacting antigen. It is important to distinguish the mechanism of the adverse response because (a) the patient may require further anaesthesia at some later date, (b) it affords a critical assessment of the advantages and disadvantages of any new anaesthetic drug and (c) there may be legal implications of death during anaesthesia.Recent investigations have suggested that certain transient changes may occur in the blood of patients who suffer an adverse reaction following the administration of an i.v. anaesthetic agent. Alterations in the concentrations of certain circulating components of the complement system, of immunoglobulin IgE and of white cell numbers have been apparent from serial sampling of blood from such patients (Watkins, Appleyard and Ward, 1975). These changes may be quantitated by simple laboratory techniques. The measurement of consumption and conversion of complement component C3 in sequential blood samples taken over 24 h following an adverse response provides a convenient laboratory assay of the adverse reactions. METHODS Blood samplesVenous blood (5 ml) was collected into a heparinized tube as soon as possible after the onset of the reaction and the time, relative to onset, was noted. Three further heparinized blood samples (5 ml) were collected at convenient times over the next 24 h. A further sample was taken not sooner than 5 days after the event to provide a "base-line" profile of the patient. The plasma was separated from these samples by centrifugation and stored at -20 to -25 °C pending analysis.