Perspectives of velocity measurements of a random bioflow and its spatial structure by speckle-methods are studied in this paper. The mechanism of output signal formation and the optical scheme of the speckle-microscope are described. Amplitude, phase and frequency characteristics of this measuring system are investigated. The main types of signal distortions in the speckle-microscope are considered. Results of the analysis of output characteristics are applied to measurements of the spatial distribution of the refractive index of a blood flow in a small single microvessel. It is shown that the minimal size of flow inhomogeneities (with fluctuations in the optical path of the investigated object, less than 0.01 lambda) that can be resolved by the speckle-microscope is about a quarter of the waist beam diameter.