The paper deals with laboratory testing of weak rocks. Circumstances influencing the laboratory research, quantity and quality of obtaining geotechnical characteristics are specified as well. For many lithological types of weak rock it is impossible to specify in advance the total range of properties which can be obtained by the laboratory research. Generally, the laboratory investigation begins by sampling and preparing the testing specimens. The choice of the test methods for determination of the geotechnical properties depends on the ability to prepare regular (prismatic or cylindrical shapes) or irregular specimens and on the stability in contact with water. The necessity of individual approach when determining geotechnical characteristics of weak rocks is documented on the example of Neogene siltstones from the territory of Slovakia that have been suggested as a host rock environment for the perspective deep geological repository of the high-level radioactive waste. They represent rock material with uniaxial compressive strength (UCS) about 30 MPa that is unstable in water. General chart for the identification of ambiguous rocks is presented in the paper. The Point load test was evaluated as suitable tests for identification and characterization of this rock type. The correlation of the Point load test index (Is₅₀) with the UCS was analysed to propose appropriate Is₅₀ to UCS conversion factor for weak rocks.