Bivalve molluscs, particularly marine mussels, are used widely as sentinel organisms in environmental quality monitoring and assessment. Biochemical and cellular responses to pollutant exposure (i.e., biomarkers) increasingly are investigated in mussel tissues and their measurements largely used in marine environmental monitoring. The aim of the present study was to investigate possible pollutant-induced morphometric alterations in Mytilus galloprovincialis granulocytes in view of future applications as a sensitive, simple, and quick biomarker for monitoring and assessment applications. Granulocyte morphometric alterations were determined by image analysis on Diff-Quick stained cells. For the first time, the rapid alcohol-fixed Diff-Quick stain protocol, utilized in clinical and veterinary applications for immediate interpretation of histological samples, was shown to be suitable for rapid cytological staining of M. galloprovincialis haemocytes. The present study was carried out in standardized laboratory conditions and further validated in field conditions. Results show consistent pollutant-induced enlargement of mussel granulocytes. This was verified by standardized biomarkers such as metallothionein concentrations in the digestive gland or lysosomal membrane stability in laboratory and field exposures. Results further suggest that the observed morphometric alterations can be used as a biomarker of chemical stress. Because no single biomarker generally is adequate for describing the complexity of effects induced by environmental pollutants on the organisms, we propose that morphometric alterations of granulocytes should be used in a biomarker battery in marine environmental monitoring programs such as Mussel Watch.