In this paper, we review our recent researches on a suspension of swimming micro-organisms. Firstly, interactions between two swimming micro-organisms are discussed. We introduce our experimental works using Paramecium caudatum, and numerical works using a squirmer model. The results show that changes in direction between two swimming cells are induced mainly by hydrodynamic forces and that biological interaction is a minor factor. Secondly, the rheological and diffusive properties of a semi-dilute suspension of squirmers are discussed. We introduce our numerical works and show that a suspension of bottom-heavy squirmers has strong non-Newtonian properties. We also show that the spreading of squirmers is correctly described as a diffusive process after a sufficiently long time, even though all the movements of the squirmers are deterministically calculated.