WHEN human lymphocytes are incubated with phytohaemagglutinin (PHA), an extract of Phaseolus vulgaris, they transform into blast cells, which enter cell division1,2. This process is accompanied by a rise in the rate of synthesis of RNA and protein3,4 and by changes in the types of RNA synthesized5,6. Actinomycin, which acts primarily by inhibiting RNA synthesis, is known to inhibit protein synthesis secondarily in many tissues, but the synthesis of some proteins has been found to be resistant to actinomycin7. This communication reports the effect of actinomycin on protein synthesis by lymphocytes.