The purpose of this paper is to report on our work to develop a real-time monitoring device by using micro-cantilevers for the mass detection of biological organisms in air and fluid. The biological agent used was Bacillus anthracis Sterne spore. The experiment was conducted using a laser Doppler vibrometer (LDV) to measure the resonant frequency of the thermal noise cantilevers and the corresponding decrease in frequency as a result of the added mass. Moreover, the added mass attributed to the spores was quantified and compared in air and deionized (DI) water. The silicon cantilevers used in this study were of lengths ranging from 25 μm to 50 μm, 200 nm thick and a width of approximately 9 pm. The first part of the experiment consisted of suspending spores onto the cantilevers in fluid, drying the cantilevers, performing measurements in air and extracting the mass of the added spores. The average mass of a spore in air was 367 fg. The second part of the experiment utilized antibody and bovine serum albumin (BSA) physically adsorbed onto the cantilevers in order to fix the spores on the surface during the measurements in deionized water. The extracted mass of a spore in fluid was measured to be an average of 1.85 pg. This study demonstrated the ability to detect biological samples not only in air but also in a liquid environment.