The level of blood natural killer (NK) cell activity was determined in relation to chlamydial infections. A group of 10 women who had recovered from chlamydial salpingitis was compared with a similar group who had had chlamydial cervicitis. Ten healthy female blood donors with no history of chlamydial infections served as controls. The spontaneous cytotoxicity of non-adherent blood lymphocytes was determined in a 3-hour assay with radiolabelled K562 cells as targets. There were no significant differences in the NK cell activity of the three groups. No correlation between NK cell activity and chlamydial IgG antibody titer in serum could be found. The level of NK cell activity as determined in this system cannot explain why some patients get a more severe form of chlamydial infection than others do.