The invention describes the use of a pharmaceutical preparation containing a trifunctional bispecific and/or trispecific having the following properties: a) binding to a T cell; b) binding to at least an antigen on a tumor cell associated with malignant ascites and/or pleural effusion; c) binding, by its Fc portion (in the case of bispecific antibodies) or by a third specificity (in the case of trispecific antibodies), to Fc receptor-positive cells for the destruction of the tumor cells in the treatment of malignant ascites and/or pleural effusion.