Usually, clay creates a suitable living environment for microorganisms, and some clays and clay materials possess antimicrobial properties that can be used, for example, in medicine and in the production of construction materials. Antimicrobial activity of natural clay and treated clay materials was studied. The results showed that the studied Latvian natural clay samples were devoid of antimicrobial activity. Porous clay ceramic granules made from quaternary deposits of Prometejs clay and sintered at the temperatures from 800 °C to 1150 °C, whole or crushed, in low concentration inhibited the growth of bacteria (Gram-negative Pseudomonas putida and Escherichia coli, and Gram-positive Staphylococcus aureus) in the aquatic environment. At higher concentration, the bactericidal effect was noticed. Suspension alone, without whole or crushed granules, had no effect on the growth or viability of bacteria. Studied ceramic materials showed no antifungal activity. Pearson correlation coefficient r = 0.84 showed a close relationship between pH value and electrical conductivity of the suspension. Attempts were made to explain the observed antibacterial effect with the formation of hydroxyl ions and increase of pH in the suspension that occurred during hydration of CaO and other oxides of alkali and alkaline earth metals. Chemically treated clay containing cationic surfactants octadecyltrimethylammonium chloride (ODTMA) and hexadecyltrimethylammonium bromide (HDTMA) showed both antibacterial and antifungal activity. Clay samples enriched with surfactants reduced the suspension pH to 2.50-2.36, increased its electrical conductivity and destroyed microorganisms. The opportunity for the development of new antimicrobial clay material technologies and products was suggested.