Microplasmodia of P. polycephalum were grown either in batch culture, in both complex and defined media to give a 3-4 fold variation in growth rate, or in a chemostate. The protein/DNA ratio of batch cultures was almost invariant, whilst the RNA/DNA ratio increased as a non-linear function of growth rate. The amount of ribosomal RNA, expressed as a fraction of total RNA, showed little variation and this was also true for the proportion of ribosomes found in polyribosomes. Calculation of the rate of protein synthesis per ribosome shows that this parameter increases by approximately 50% over the range of growth rates studied, although it should be emphasized that the effect of protein turnover has not yet been taken into account. Enrichment of batch cultures growing in a defined medium produced an increase in the rate of RNA synthesis. Data obtained with chemostat cultures differed in several respects from those described above for batch cultures, especially at low growth rates, and are discussed in relation to the early stages of differentiation of microplasmodia to spherules.