Test of the stability and activity of use-dilutions of a disinfectant. One tube (A) of a pair of identical dilutions is inoculated with a suspension of Ps. aeruginosa on Day 1, leaving it and the uninoculated tube (B) at room temperature for 7 days. On Day 8, 5 subcultures of tube (A) are made into nutrient broth, and tube (B) is inoculated with a fresh suspension similar to that used for tube (A) the week before. Tube (B) now stands for a further 7 days, and on day 15 is sub-cultured to nutrient broth.