Pseudomonas cepacia strain 224c was subjected to treatment with two alternating cycles of penicillin and D-cycloserine in order to isolate spontaneous temperature-sensitive (ts) mutants. Of 150 surviving clones tested, ten were ts mutants; these were partially characterized, and one, Pcp-5, was chosen for experiments to determine the in vivo growth rate of the wild-type (wt) and clearance from the peritoneal cavities of mice. This mutant ceases growth almost immediately after transfer from 29-degrees-C to the nonpermissive (36-degrees-C) temperature and has a reversion rate of 10(-8). In vivo experiments were performed by inoculating mixtures of Pcp-5 and wt into the peritoneal cavities of ICR mice. The ts mutant was cleared quantitatively (90% by 4 h), and the mean generation time (MGT) of the wt in the murine peritoneal peritoneal cavity was 94 min.