The immune system of crustaceans consists of immune cells and molecules whose main function is to distinguish between self and non-self and to maintain an adequate response to a potential threat to the organism. In the haemolymph of control or asimptomatic terrestrial isopods Porcellio scaber, we optimized and evaluated reference values of cellular and humoral parameters indirectly or directly involved in the function of the innate immune system. Using a combination of light and electron microscopic techniques, we have described three main haemocyte types (semigranulocytes, granulocytes, hyalinocytes) in the haemolymph of asimptomatic P. scaber and provided information on their morphology and ultrastructure. Differential interference contrast microscopic tehnique was recognised as a rapid and reliable method for measuring the density of different haemocyte types in fresh haemolymph samples from asimptomatic animals, with semigranulocytes being the most common type (65% on average), followed by hyalinocytes (18%) and granulocytes (17%). The performance of the method optimization for measuring cellular and humoral parameters in the haemolymph of P. scaber was validated with the evaluation of the values of selected parameters in the haemolymph of animals with evident symptoms of bacterial (Rhabdochlamydia porcellionis) and viral (Iridovirus IIV-31) infection. The values of measured parameters in the haemolymph of symptomatic P. scaber differ significantly from asymptomatic P. scaber. Isopods respond differently to bacterial or viral infection, i. e. different set of innate immunity processes are induced in each of the infection. While bacterial infection triggers significant changes in humoral parameters, cellular parameters were significantly altered upon viral infection.