SUMMARY Dictyostelium discoideum wild-type amoebae incorporate 3H-uridine into RNA at a linear rate during the developmental sequence until the terminal stage of fruiting body construction. However, 3 aggregateless strains perform in similar fashion (except for the final decrease). A sensitive criterion of bacterial contamination, making use of MAK-column chromatography of the labelled RNA, is described. Organisms and experimental conditions. Wild-type Dictyostelium discoideurn strain NC-4 (haploid) and aggregateless mutant derivatives were grown in association with Aerobacter aerogenes on SM agar (Sussman & Lovgren, 1965). The amoebae were harvested at room temperature in SM broth and streptomycin sulphate (0.5 mg./ml.), spun at 1OOOg for 2-3 min. and resuspended in the SMfstreptomycin broth at a concentration of 2 x lo7 amoebae/ml. After incubation for 1 hr on a shaker at 22", the amoebae were harvested by centrifugation, washed once with cold water and * Present address: 1 Chome 13-11 Daiz AWA, Setagaya-Ku, Tokyo, Japan.