A protein such as an enzyme of antibody is immobilized by crosslinking crystals of the protein with a multifunctional crosslinking agent. The crosslinked protein crystals may be lyophilized for storage. A preferred protein is an enzyme such as thermolysin, elastase, asparaginase, lysozyme, lipase or urease. Crosslinked enzyme crystals preferably retain at least 91% activity after incubation for three hours in the presence of a concentration of Pronase.TM. that causes the soluble uncrosslinked form of the enzyme to lose at least 94% of its initial activity under the same conditions. A preferred enzyme:Pronase.TM. ratio is 1:40. Enzyme crystals that are crosslinked may be microcrystals having a cross-section of 10.sup.-1 mm or less. Crosslinked enzyme or antibody crystals may be used in an assay, diagnostic kit or biosensor for detecting an analyte, in an extracorporeal device for altering a component of a fluid, in producing a product such as using crosslinked thermolysin crystals to produce aspartame, in separating a substance from a mixture, and in therapy.