In order to compare the prevalence of atopy and bronchial hyperreactivity among Papua New Guinian (P.N.G.) and Australian populations, skin prick tests and methacholine bronchial challenge tests were performed. A civilian and an army population from each country were examined and those with past or present asthma, recent respiratory tract infection and chronic lung disease were excluded. No statistical difference in the prevalence of atopy was found between the four populations. In the P.N.G. population 40 and 49%, and in the Australian population 27 and 39%, were found to be atopic, without symptoms of past or present allergic disease. The house dust mites were the commonest allergens in all populations. In response to methacholine (0·3 mg), only 6% of subjects had falls in 1 sec forced expiratory volume (FEV1) of more than 12% (upper limit of normal range) and only two were in the asthmatic range. There was no correlation between the degree of bronchial hyper-reactivity and atopic status; however, the degree of bronchial hyperreactivity was slightly greater in the New Guinea civilian than in the Australian civilian population. In the absence of asthma, atopic status does not appear to cause increased bronchial reactivity, suggesting that some factor other than atopy must be present for the development of bronchial hyperreactivity characteristic of asthma.