In the usual serologic test for Mycoplasma hominis, disks containing antibody to a prototype strain are applied to an agar plate that has been inoculated with the microorganism to be tested, and inhibition of growth is studied. The recent demonstration that M. hominis is not serologically homogeneous but consists of at least seven serotypes suggested the need for reexamination of the specificity of the disk test. The inhibition of growth of M. hominis on solid medium by disks containing seven antisera to the seven prototype strains was studied. Antisera to strains 1, 2, and 3 significantly inhibited all seven prototype strains, but the other four antisera did not inhibit the growth of all heterologous strains. The individual antisera 1, 2, and 3 did not inhibit all of a group of 19 recent genital isolates of M. hominis. However, when the three antisera were combined, they inhibited all 19 isolates as much as or more than any of the three alone. The growth of the prototype strain 5 (PG 21) and of some recent clinical isolates was not completely prevented by the homologous or reference antisera, but the growth of all was partially inhibited.