Monosubstituted adenosine 5'-triphosphate (ATP) derivatives with a substituent of up to four atoms at any of eight positions in the adenosine moiety, or with an isosteric group replacement at O5' or in the triphosphate moiety, have been evaluated kinetically as substrates and inhibitors of liver (I), kidney (II), and Novikoff hepatoma (T) variants of rat methionine adenosyltransferase. Inhibitory potencies were expressed as KM(ATP)/Ki (for competitive inhibition vs. ATP) or as KM(ATP)/KM when no Ki value was available. Variant I was inhibited more powerfully than II or T by all of four ATP derivatives for which comparative data were obtained. Among 15 ATP derivatives, four were substrates of II or T and the remainder inhibited II and T competitively with respect to ATP; most derivatives exhibited at least moderate (greater than 0.5) inhibitory potency. Differential inhibition of II and T was shown by 11 of 14 ATP derivatives; relative inhibitory potencies (T:II) ranged from 5.5 with 2-SCH3-ATP [KM(ATP)/Ki = 1.3 with T] to 0.24 with the ATP isostere with a C5'-CH2-P alpha system [KM(ATP)/Ki = 1.9 with II]. The most effective inhibitor was the P beta, P gamma imido isostere of ATP with inhibitory potencies of 25 and 35 for II and T, respectively. The findings provide further evidence that substrate derivatives with single short groups attached at various positions, or with single isosteric group replacements, are frequently useful probes in the design of isozyme-selective inhibitors.