A null mutation in the SPE2 gene of Saccharomyces cerevisiae, encoding S-adenosylmethionine decarboxylase, results in cells with no detectable S-adenosylmethionine decarboxylase, spermidine, and spermine. This mutant has an absolute requirement for spermidine or spermine for growth; this requirement is not satisfied by putrescine. Polyamine-depleted cells show a number of microscopic abnormalities that are similar to those reported for several cell division cycle (cdc) and actin mutants. These include a striking increase in cell size, a marked decrease in budding, accumulation of vesicle-like bodies, absence of specific localization of chitin-like material, and abnormal distribution of actin-like material. The absolute requirement for polyamines for growth and the microscopic abnormalities are not seen if the cultures are grown under anaerobic conditions.