In order to probe the apparent biosynthetic similarities between nargenicin and erythromycin, we investigated the incorporation of (2S,3R)-[2,3-¹³C₂]-2-methyl-3-hydroxypentanoyl NAC-thioester (1) into nargenicin. Ten 70-mL fermentation cultures of Nocardia argentinensis were incubated with labeled 1, and the resulting nargenicin was extracted, purified, and analyzed by ¹³C NMR. The spectrum showed enhanced and coupled doublets at 32.76 and 78.79 ppm (corresponding to C-16 and C-17, Jcc=36.2 Hz), establishing the intact incorporation of 1. This indicates the polyketide synthetase of N. argentinensis can utilize a partially elaborated intermediate of the chain elongation process, consistent with a scheme involving adjustment of functionality and stereochemistry of the growing polyketide chain prior to each condensation reaction. The observed incorporation of the (2S,3R)-enantiomer of 1 aligns with the previously determined absolute configuration of nargenicin. Incorporation experiments involving more advanced intermediates of the chain-elongation process are in progress.