Abstract: The invention relates to a method of assembling a cap (20) of a closure device (9) with an open end (6, 7) of a housing container (5) for forming a container system (1) for body fluids, tissue parts or tissue cultures, whereby a relative rotating or pivoting movement is effected between the cap (20) and the housing container (5) about a common longitudinal axis (14). In order to generate the relative movement, a pressing force (F) is applied to at least one of the components to be assembled directed more or less in the direction of the longitudinal axis (14). The invention further relates to a cap (20) for forming the closure device (9), a housing container (5) and a container system (1) obtained as a result, comprising at least the cap (20), a sealing device (21) device retained in it and the housing container (5).