Abstract: Method of and apparatus for monitoring a waste influent stream to a waste treatment biological basin which contains active microorganisms for levels of materials that are toxic or otherwise injurious to the microorganisms in the basin involving the (a) measurement of the rate of oxygen uptake, R, in a first sample of mixed liquor from said basin, the first sample having a predetermined volume and a predetermined dissolved oxygen content; (b) measurement of the rate of oxygen uptake, M, in a mixture of a second sample of mixed liquor taken from said basin and a sample of liquid from said waste influent stream, the second sample of mixed liquor having substantially the same volume and dissolved oxygen content as said first sample; (c) computation of the differential, M-R; (d) provision of a toxic condition signal when said differential, M-R, is lower than a predetermined value indicating a toxic effect of the influent stream sample on the sample of mixed liquor from the basin; and/or provision of an undue oxyge