1993. Isopropyl cinodine and nalidixic acid were compared in the direct viable count. With raw water and biofilms, elongated cells were seen in the presence of isopropyl cinodine. Increased incubation time led to an increased direct viable count. Individual bacteria responded differently to isopropyl cinodine. Five organisms grew in the presence of 0.01 p g ml-' of isopropyl cinodine but were inhibited by 0.1 pg ml-'. These values for a sixth organism were 0.1 pg ml-' and 1.0 p g ml-' respectively. T h e direct viable count was done with inocula taken when the cells were in either lag, log or stationary phases of growth. No differences were seen in the percentage of elongated cells within an experiment but there was variation between experiments. T h e effect of nalidixic acid and isopropyl cinodine appeared to be additive with respect to inhibition of growth, but little or no additive effect was seen upon the percent of nutrient responsive cells. Environmental Microbiology 53, 2889. Zelibor, J.L., Doughten, M.W., Grimes, D.J. and Colwell, R.R. (1987) Testing for bacterial resistance to arsenic in monitoring well water by the direct viable counting method. Applied and Environmental Microbiology 53, 292!+2934. Zimmerman, R., Iturriaga, R. and Becker-Brick, J. (1978) Simultaneous determination of the total number of aquatic bacteria and the number thereof involved in respiration. Applied and Environmental Microbiology 36, 926-935.