In order to generate further characterisation data for the lyophilised product Erwinia chrysanthemi L-asparaginase, reconstituted drug product (DP; marketed as Erwinase or Erwinaze) was analysed for subvisible (2-10 μm) particulate content using both the light obscuration (LO) method and the newer flow-imaging microscopy (FIM) technique. No correlation of subvisible particulate counts exists between FIM and LO nor do the counts correlate with activity at both release and on stability. The subvisible particulate content of lyophilised Erwinia L-asparaginase appears to be consistent and stable over time and in line with other parenteral biopharmaceutical products. The majority (ca. 75%) of subvisible particulates in L-asparaginase DP were at the low end of the measurement range by FIM (2-4 μm). In this size range, FIM was unable to definitively classify the particulates as either protein or non-protein. More sensitive measurement techniques would be needed to classify the particulates in lyophilised L-asparaginase into type (protein and non-protein), so the LO technique has been chosen for on-going DP analyses. E. chrysanthemi L-asparaginase has a lower rate of hypersensitivity compared with native Escherichia coli preparations, but a subset of patients develop hypersensitivity to the Erwinia enzyme. A DP lot that had subvisible particulate counts on the upper end of the measurement range by both LO and FIM had the same incidence of allergic hypersensitivity in clinical experience as lots at all levels of observed subvisible particulate content, suggesting that the presence of L-asparaginase subvisible particulates is not important with respect to allergic response.