Methotrexate (MTX) is widely used in the treatment of a number of oncological and hematological diseases. Due to its known limitations, MTX is often conjugated with different carriers to obtain amended forms of the drug. In this study, the potential influence of the substitution level (loading ratio) of the dextran T10- and T40-based MTX conjugates (D-MTX) on their properties were investigated in vitro and in vivo. The clear dependence of the in vitro antiproliferative effect on the substitution level was established only in the case of the dextran T10-based preparations (T10-MTX conjugates). Conjugates with the higher substitution level had the lower antiproliferative effect. For the dextran T40-based (T40-MTX conjugates) set no similar relationship was observed in the tested range of substitution levels, nor was any dependence observed between the biological properties of the D-MTX preparations in vivo and their substitution levels. However, the difference between the two conjugates was well pronounced in a multiple-dose schedule, when the advantage of T40-MTX over T10-MTX was cumulative during the prolonged course of administration.