Escherichia coliUB1005 (DCO), an envelope mutant (DC2),Pseudomonas aeruginosa799 and an envelope mutant (799/61) were exposed to sodium deoxycholate (DOC), sarkosyl and sodium lauryl sulphate (SLS). DOC was the least effective lytic agent, but the twoPs. aeruginosastrains, especially 799/61, were more susceptible to DOC and sarkosyl than theE. coliones. SLS was an efficient lysing agent, although Ps.aeruginosa799 was the least susceptible of the four strains. DC2 was lysed more rapidly and to a greater extent than UB1005 by all three agents. The mutant strains, especially DC2, were more sensitive to selective media than the wild‐type one