Simultaneously with the development of vaccines against diphtheria, the authors and their colleagues have studied the biological properties of the causative agent, Corynebacterium diphtheriae, and its role in the development of epidemics. This study has been made possible by the development of a lysotyping scheme for the bacterium, covering all three biotypes-gravis, intermedius and mitis. On the basis of this scheme it was possible to lysotype 73% of the 8694 strains of C. diphtheriae isolated in Romania between 1956 and 1966-99% of the gravis strains, 39% of the intermedius and 19% of the mitis; in all, 19 lysotypes were distinguished.To check the applicability of the lysotyping scheme on an international scale, 395 strains of C. diphtheriae were obtained from the reference laboratories in 14 countries in four continents. It was possible to lysotype 62% of the strains, including 93% of the gravis type. Details are given of the geographical distribution of the different lysotypes; two lysotypes not isolated in Romania were encountered among the foreign strains and will be incorporated into the scheme as the 20th and 21st lysotypes.There is a need to extend the number of intermedius- and mitis-type phages to enable more of these types of strain to be distinguished.