The binding of p-aminobenzoyl-L-glutamate and 2,4-diaminopyrimidine to dehydrofolate reductase from Lactobacillus casei MTX/R in the presence of a series of co-enzymes and coenzyme analogues has been measured fluorometrically. These two ligands, which can be regarded as "fragments" of the powerful inhibitor methotrexate, have been shown to bind cooperatively in the absence of coenzyme [Birdsall, B., Burgen, A. S. V., Rodrigues de Miranda, J., & Roberts, G. C. K. (1978) Biochemistry 17, 2102], p-amino-benzoyl-L-glutamate binding 58 times more tightly in the presence of 2,4-diaminopyrimidine than in its absence. In the presence of coenzymes, this cooperativity ranges from 1.8- to 428-fold. The effects of coenzymes on individual binding steps range from an 8-fold decrease in binding constant to a 23-fold increase. The structural specificity of these effects are discussed in terms of a model involving ligand-induced conformational changes and compared with the effects on trimethoprim and methotrexate binding described in the preceding paper [Birdsall, B., Burgen, A. S. V., & Roberts, G. C. K. (1980) Biochemistry (first paper of four in this issue)].