Bacterial activity and growth were monitored by following the changes of electrical impedance of cultures in liquid media. The signal is expressed automatically as a curve similar to grwoth curves produced by other methods. The technique offers a new, rapid and sensitive means of detecting active micro-organisms and is potentially the basis of rapid automated systems in this field. The impedance changes indicate that the micro-organisms metabolise substrates of low conductivity into products of high conductivity and that the changes are due to the activity of the micro-organisms rather than increase in their numbers. The activity of strains of Escherichia coli, Klebsiella aerogenes, Pseudo-monas aeruginosa, Staphyloccus aureus, and Streptococcus faecalis was detected within 2 h with inocula of 10-3 minus 10-5 organisms per ml. Different reactions of bacteria in various media suggest that the method may be applied to the rapid identification of micro-organisms. The inhibitory effect of antibiotics on bacteria was demonstrated within 2 h, indicating that the method may be useful for the rapid determination of bacterial sensitivity to antibiotics and the rapid assay of antibiotics in serum. Correlation of response time to initial inoculum allows estimatin of numbers of viable organisms. The sensitivity of the method allowed detection of activity due to Myoplasma argininii within 3 h; this suggests that the method might be applicable to the rapid detection of other slowly growing organisms, such as mycobacteria.