In several studies lysozyme has been employed as a model protein to investigate the effects of formulation factors upon biological activity. The aim of this work was to develop and validate an HPLC technique to assay lysozyme and to compare the results with biological activity determined from a validated turbidimetric assay. The turbidimetric assay was based upon the lytic action of lysozyme on Micrococcus lysodeikticus cells, whilst the reverse-phase HPLC assay employed an acetonitrile gradient in 0.1% trifluoroacetic acid. The limits of detection and quantification were 3.84 and 6.24 microg mL(-1) for HPLC assay, whilst the corresponding values for turbidimetric assay were 1.94 and 3.86 microg mL(-1). The methods were used to monitor the loss of enzyme activity after heating. Lysozyme concentrations determined from HPLC peak height were found to correlate (r2 = 0.9963) with those obtained from turbidimetric assay.