A comparative study of the viability of 34 lots of freeze-dried BCG vaccine has been carried out using an oxygen electrode polarographic technique, Warburg respirometry and colony counting. There were no statistical differences between the results obtained with the polarographic and Warburg techniques. Both methods gave reliable and similar results and showed a positive correlation (r = 0.8615). Comparison between the polarographic and colony-counting methods showed a positive correlation (r = 0.6530); for comparison between the Warburg and colony-counting methods, the correlation value was r = 0.6868. All the tests were significant at the level of alpha = 0.05. The advantages of the polarographic technique are that it is much less time- and material-consuming than other methods. It is a reliable, inexpensive and convenient method for BCG vaccine quality control.