It paved the way for the discovery and diagnosis of microscopic pathogens, such as malarial parasites,[2] and later developments of new histological stains that became fundamental to microbiology and physiology.

[3] While working on his doctoral research, Romanowsky developed the first effective staining method for malarial parasite in 1890.

British zoologist and science historian, Francis Edmund Gabriel Cox remarked the discovery as a serendipitous case that became "one of the most significant technical advances in the history of parasitology.

[15] By mixing specific amount of eosin and methylene blue, Romanowsky found that the mixture gave images of contrasting clarity that helped to visualise different parts and components of cells.

[3][17] The chemical phenomenon by which a mixture of stains produces vibrant cell images is known as "Romanowsky effect".

[15][18] In December 1890, Romanowsky published his invention as a preliminary report of his major work for his doctoral thesis in the journal Vrach as "On the question of the structure of malaria parasites" (as translated in English).

Romanowsky discovered that instead of fresh methylene blue, an aged and mouldy solution gave the best result, while eosin should be free of any contamination.

Cytoplasm in eosinophils is saturated-pink, whilst that in the malaria parasite and lymphocytes is light blue.

[8] The staining method remains the "gold standard" for visualising blood samples, especially for malarial infection,[2] and in immunohistochemical studies.