The role of conformational changes and subunit interactions in the allosteric mechanism of aspartate transcarbamoylase was evaluated by studying hybrid enzyme molecules containing cross-linked subunits. Native enzyme was cross-linked with tartryl diazide in the presence and absence of substrate analogues. The two types of modified enzyme derivatives were each dissociated into catalytic (c3) and regulatory (r2) subunits. Hybrids were constructed with modified catalytic subunits and unmodified regulatory subunits or vice versa. Subunits from different derivatives also formed hybrids.All hybrids containing cross-linked catalytic subunits showed hyperbolic substrate saturation curves while cross-linking in the regulatory subunit alone did not abolish cooperativity. The type of cross-linking in the catalytic subunit had a decisive influence on the substrate affinity of the hybrid as well as its response to the allosteric effectors ATP and CTP. However many effects were also dependent on the presence of regulatory subunits. The results implicate a substantial conformational change in the catalytic subunit upon substrate binding and suggest an important role for the c–r interaction in the allosteric mechanism.