AbstractWe have covalently attached pure chicken H4 and M4 lactate dehydrogenase to porous glass beads by a variety of chemical methods. The attachment of these isoenzymes by the glutaraldehyde and diazo linkages generates immobilized enzymes that are stable for months, show decreased substrate (pyruvate) inhibition and exhibit a pH‐rate profile independent of pH. These insolubilized catalysts exhibit apparent Km values similar to those of the native enzymes. A comparison of the glutaraldehyde‐ and aryl glass‐bound isoenzymes indicates that the former linkage allows for greater stability toward pH and heat denaturation and is also more resistant to attack by proteolytic enzymes. There is also evidence that in the presence of 5.3 M lithium chloride no subunits are dissociated from the glass‐bound enzymes. Upon renaturation, the glutaraldehyde‐glass‐bound enzyme affords an almost quantitative recovery of activity, while the native and aryl‐glass‐bound enzymes give approximately thirty per cent recovery. The degree of antibody inhibition exhibited by the immobilized enzymes was also examined.