Groups of school children aged 6-14 years were tested with PPDS and one of the following five antigens simultaneously, namely PPDA, PPDF, PPDG, PPDPL and PPDY, PPDS was prepared from the human tubercle bacillus, and the others were prepared from M. avium, M. fortuitum, the 'Gause' organisms, M. marinum and M. kansasii respectively. It was observed that there was some increase in induration size to PPDA, PPDG, PPDL, PPDS and PPDY with increase in age, while M. fortuitum gave a preponderance of small reactions with no increase of size with age. It was also observed that there was evidence of cross sensitization between PPDS and all the other antigens, particularly in those who had negative or doubtful reactions.