Dextrans were first used in clinical practice in 1947. Raw dextran is produced by bacterial conversion of sucrose to a branched chain structure which is heterogeneous in composition. Further refinement is carried out by hydrolysis and fractionation. Clinically useful dextrans form only 1 percent of the total molecular spectrum of raw dextran. The two most frequently used dextrans are Dextran 40 and Dextran 70. These have molecular weights of 40,000 and 70,000 respectively. The use of average molecular weight to describe dextrans is generally unsatisfactory since an infinite variety of dextrans could meet a particular weight specification. Categorization of dextrans will be discussed in detail. Clinical uses include plasma expansion, antithrombogenesis and haemodilution. Major efforts to reduce the antigenically active components of clinically used dextrans have been made and manufacturers have identified the principal factors responsible for allergic responses. However, major allergic responses to dextran still cause clinicians great concern and the problem is by no means resolved.