A temperature-sensitive mutant of Neisseria gonorrhoeae strain tsA-1 was used in a transformation test for the laboratory diagnosis of gonorrhea. This transformation test (C test) is based on a spotting of a DNA lysate, obtained through simple base-acid extraction of a cervical-swab specimen, onto a lawn of tsA-1, which is then incubated for one to two days at 37 C. Of 1,053 cervical-swab specimens tested, 52 contained the gonococcal DNA necessary for restoring the ability of tsA-1 to grow well at 37 C; 49 of the 52 specimens were identified as N. gonorrhoeae by routine diagnostic laboratory procedures. The time required for the C test is two to three days, which is shorter than that required for routine diagnostic laboratory tests, and the C test involves a minimum of common laboratory supplies and expertise. The discrepancies between results of the C test and routine procedures are explained as follows. In one case an isolate suspected to be N. gonorrhoeae was nonreactive in the sugar fermentation test, and in two cases the agar plates were overgrown with yeasts; neither situation affected the C test.