To provide more information on the 5-aminolaevulinic acid (ALA)-induced photodynamic effect on nucleoli, morphologically expressed nucleolar asynchrony (the presence of 'active' large nucleoli with an uniform distribution of RNA and 'resting' ring-shaped nucleoli in one and the same nucleus) was studied in cultured HL-60 leukemic granulocytic precursors using a simple cytochemical procedure for the demonstration of RNA. Nucleolar asynchrony was mainly expressed in cells which were apparently resistant to ALA-based photodynamic treatment (PDT) since most of them (about 75%) exhibited this phenomenon.