1. Several agar derivatives were prepared, among them Davis Girard T agar with a cathodic relative endosmotic liquid flow rate about 30% lower than that of agarose; and DEAE-Bacto agar with a reversed, strongly anodic relative endosmotic flow rate of +9.3·10−5cm2·V−1·sec−1. 2. Trimethylaminoacethydrazide- (Girard T agar), pyridiniumacethydrazide- (Girard P agar) and diethylaminoethyl- (DEAE-agar) groups were introduced into Bacto agar polysaccharides in numbers of 0.17, 0.16 and 1.72 per 10 hexose units respectively. 3. The use of Davis Girard T agar made immuno-osmophoresis applicable to antigens with negative net charges too low to overcome the endosmotic liquid flow in agarose. 4. The still considerable endosmotic liquid flow in agarose was explained as resulting from acidic constituents, among them possibly pyruvic acid. 5. HIO4 treatment of agar suggested that native agarose may have in some of its l-galactose units a diol configuration in position 2,3, with the 6-position taking part in a sulfate bond, instead of 3,6-anhydro ring configuration.