A shampoo composition for the treatment of dandruff comprises dimethyl sulphoxide, a detergent selected from cationic, non-ionic, e.g. polyoxyalkyl ethers and anionic, e.g. alkyl aryl sulphonates and sodium lauryl sulphate, types and if desired water, perfume and acceptable colorants.ALSO:The growth of a vegetative form of fungi, bacteria or yeasts is stimulated while spore formation is depressed by contacting the microorganisms with an effective amount of a composition containing as an essential agent a dialkyl sulfoxide in which each alkyl group contains 1 to 4 carbon atoms and a carrier. Preferably less than 3% by weight of dimethyl sulfoxide is present in a culture of the fungi, bacteria or yeasts. Stimulation of vegetative form growth results in greater yields of certain products e.g. antibiotics such as penicillin, alcohol and acetone.ALSO:Microorganism (i.e. bacteria, fungi, virus and yeast) growth is retarded by contacting the microorganisms with an effective amount of a composition containing, as an essential agent, a dialkyl sulphoxide in which each alkyl group contains 1 to 4 carbon atoms, and a carrier. Preferably, the above composition also contains an effective amount of a known agent for retarding the growth of microorganisms, dimethyl sulphoxide being present in an amount sufficient to increase the sensitivity of the microorganism to the action of the agent. For agents known to retard the growth of microorganism, but to which the microorganism was previously resistant, e.g. phenyl mercuric acetate, the amount of dimethyl sulphoxide is sufficient to cause the microorganism to become sensitive to the agent and thereby retard growth. Compositions for retarding the growth of (1) plant viruses and (2) fungi respectively comprise (1) an antiviral agent, e.g. 6-mercaptopurine, and an above-defined dialkyl sulphoxide in an amount sufficient to increase the sensitivity of the virus to the antiviral agent and (2) a fungicide, a dialkyl sulphoxide and a carrier, the dialkyl sulphoxide being present in an amount sufficient to increase the sensitivity of the fungus to the fungicide or to render the fungus sensitive to a fungicide to which it was previously resistant. The preferred dialkyl sulphoxide is dimethyl sulphoxide and water is preferably used as the carrier which may optionally be present in the antiviral composition. The compositions may be used industrially in general slime control, control of growth of microorganisms in recycled water and in air conditioning units and in the protection of fibres in the textile industry. Plant viral growth is retarded by contacting the affected plant with dimethyl sulphoxide by xylem injection, soil drench or topical application of an aqueous spray solution containing 2% by weight dimethyl sulphoxide. Other dialkyl sulphoxides specified are methyl, ethyl diethyl methyl n-propyl and dibutyl sulphoxides. A shampoo composition for the treatment of dandruff comprises dimethyl sulphoxide (active agent), a detergent selected from cationic, non-ionic (e.g. polyoxy alkyl ethers) and anionic (e.g. alkyl aryl sulphonates and sodium lauryl sulphate) types and if desired water, perfume and acceptable colorants.