Escherichia coli possesses regulatory mechanisms that coordinate cell growth with the synthesis of essential macromolecules (protein, RNA and DNA). While fundamental differences have been identified in the growth habit and chromosome structure of E. coli and Streptomyces, little is known about these regulatory mechanisms in filamentous bacteria. This paper reports on the relationship between the macromolecule content of S. coelicolor A3(2) and its specific growth rate. The protein, RNA and DNA contents (g per 100 g biomass) of S. coelicolor A3(2) grown in steady-state continuous culture over a range of specific growth rates (0.025-0.3 h-1) were 31-45, 10-22 and 3.5-4.5% (w/w), respectively. This composition is qualitatively similar to that of other microorganisms. Changes in the macromolecular content of S. coelicolor A3(2) and E. coli B/r with specific growth rate appear to be essentially similar. However, the data indicate that the RNA content of S. coelicolor A3(2), grown under the conditions used, exceeds that of E. coli grown at the same specific growth rate. The data also suggest that overlapping rounds of replication are not a feature of DNA synthesis in S. coelicolor A3(2). This may be a function of the organism's low maximum specific growth rate. Alternatively, it may be a consequence of regulatory mechanisms which act to inhibit the initiation of DNA synthesis in a linear chromosome which is already undergoing replication.