According to the invention there are provided derivatives of a C21-deoxy ansamycin or salt thereof which contain a 1-hydroxyphenyl moiety bearing at position 3 an aminocarboxy substituent, in which position 5 and the aminocarboxy substituent at position 3 are connected by an aliphatic chain of varying length characterised in that the 1-hydroxy position of the phenyl ring is derivatised by an aminoalkyleneaminocarbonyl group, which alkylene group (which may optionally be substituted by alkyl groups) has a chain length of 2 or 3 carbon atoms, a phosphoric acid, or a phosphoric acid ester (such as an alkyl ester) group, or a salt thereof, and which derivatising group increases the water solubility and/or the bioavailability of the parent molecule. Such compounds are useful in therapy eg in the treatment of cancer and B-cell malignancies.