Sera from patients with gonorrhoea and from control groups were investigated by a passive haemagglutination test in which sensitizing antigen was prepared from ten isolates of N. gonorrhoeae by the phenol/ water method. Positive titres (1 in 128 or greater) were found in 82 per cent. of women and 51 per cent. of men with active gonorrhoea. In adult control groups the frequency of positive titres ranged from 8-1 and 24 per cent. An identical technique using sensitizing antigen from a single isolate of N. gonorrhoeae was considerably less sensitive. Persistence of high agglutination titres in women after treatment was demonstrated and the inability of the test to distinguish active from past infection emphasized.