Recently, also detergents have been proposed for use in the pretreatment of clinical material for cultural demonstration of Mycobacterium tuberculosis. One of such substances is the anionic detergent. Neckal BX (diisobutyl naphthaline sulphonate) which in combination with 0.5% sodium hydroxide, is capable of homogenization and decontamination of the material within 16-18 h of action. At the same time, the mycobacteria are accumulated in the sediment by precipitation in the presence of barium, calcium, and phosphate ions. The results obtained by the new method were compared with those obtained by the sulphuric acid method, in a paralles study of samples of sputum, gastric juice, and faeces and a second one of sputum samples only. By using two different formulas for reagents (Table 1), the mycobacterial isolation rate was shown to be dependent upon the concentration of the precipitant. By the following criteria, the Nekal method was superior over pretreatment with sulphuric acid: 1. The reduction of the number of contaminated was obvious in the cases of sputum and gastric juice samples and significant for sputum samples. (Tables 2-7). 2. Using Nekal BX, 31 out of 616 sputum samples were found to be positive; using sulphuric acid, their number was only 22. This difference was found to be statistically significant. The additional yield came primarily from material containing only few mycobacteria and samples which could not be assessed because of contamination present after pretreatment with sulphuric acid (Table 8). The average period which passed until reading of the cultures was approximately the same: 4.2 weeks for sulphuric acid and 4.4 weeks for Nekal (Table 9). When applying the new method, the material admits of mechanical shaking and need not be centrifuged. No strict control of the period of action is required. Taking into account these operational advantages, the Nekal method is considered particulary suitable for laboratories receiving high numbers of samples.