A comparative study of cyathin production by species and strains of the fungus Cyathus was made. Various haploid strains of C. helenae produced cyathin, though others produced no detectable amounts. The cyathin-producing strains, however, did not differ to any appreciable extent in the composition of the cyathin complex as shown by chromatographic analysis. Sectors of C. helenae produced different amounts of cyathin. All three species of Cyathus, C. africanus Brodie, C. helenae Brodie, and C. striatus (Huds.) wild. ex Pers., produced antimicrobial substances. Cyathus africanus is reported for the first time to possess biological activity.