This text outlines one of the vast and fruitful fields in protozoan microbiology and formulates the principles of obtaining biologically active substances from cultures of protozoa. The correlation between composition of selected flagellates' cell components (polyunsaturated fatty acids, membrane surface glycophospolipids, intercellular polysaccharide, b-1, 3-glycan) and their biological effects in experimental animals, is the main topic of the book. As the book's content shows, the protozoa are gradually acquiring their place among traditional producers of biologically active substances. The row of producers of BAS can be extended by including non-parasitic representatives of the protozoa (the phylum Sarcomastigophora). Representatives of the phylum Ciliophora are emerging. The book is addressed to undergraduate and postgraduate students and young scientists to provide orientation in the field, especially in the sequence of studies which lead to obtaining biologically active substances and primary evaluation of their biological effects. It may also be of interest to pharmacologists and manufacturers.