Maximum recovery of mycobacteria from clinical specimens requires an efficient decontaminating agent which is not harmful to mycobacteria. None of the methods described to date will permit survival of all the mycobacteria in a specimen while simultaneously destroying the contaminants. Studies comparing (1) Mycobactosel, Lowenstein-Jensen medium (with and without penicillin in the inoculum), and 7H10 medium using clinical specimens for (a) rate of isolation of acid-fast organisms, and (b) rate and types of contamination; and (2) Mycobactosel and 7H10 medium for rate of growth and size and morphology of colonies of stock cultures of mycobacteria and Nocardia asteroides are reported. These studies show that the contamination rate is significantly reduced on Mycobactosel and the recovery rate of mycobacteria is equal to or better than that on Lowenstein- Jensen medium, with or without the use of penicillin in the inoculum, and on 7H10. Growth appears about one week later on Mycobactosel than on 7H10, although colonial morphology at maturity is similar to that seen on 7H10. To provide both rapid growth and maximum recovery of acid-fast organisms in routine culturing of clinical specimens, it is recommended that a nonselective medium, such as 7H10, be used in conjunction with a selective one, such as Mycobactosel.