The use of different tests of airways obstruction to measure responses to inhalation of antigen by twenty-five asthmatic patients is described. On all occasions but one, when there were symptoms there was a significant fall in either vital capacity (VC), forced expiratory volume in one second (FEV1)or peak expiratory flow rate (PEFR). In no case without symptoms was there a significant fall in maximum expiratory flow rates, specific conductance or spirometric tests. It is unlikely that the use of maximum expiratory flow curves or plethysmography have any advantages over spirometry or peak expiratory flow rate in recording reaction to inhalation challenge perse, though they may show greater per cent changes from the initial value than these simpler tests. The possible reasons for this are discussed.