The properties of creatine kinase isolated from bovine heart mitochondria in dimeric (Mr = 84 +/- 6 kD) and octameric (Mr = 340 +/- 17 kD) forms were compared with those of the earlier described hexameric form of the enzyme (Mr = 240 +/- 12 kD). The kinetics of SH-group modification by DTNB, the inactivation kinetics as well as the number of modified SH-groups point to significant differences between the three oligomeric forms of the enzyme. Each subunit of creatine kinase was found to possess one "fast" essential cysteine residue whose modification by DTNB and iodoacetamide led to enzyme inactivation. The formation of an analog of the transition state complex (E--MgADP--NO3--creatine) was paralleled with partial protection of only the "fast" cysteine residue which manifested itself in the decrease of the rate of its interaction with DTNB in all the three oligomeric forms. Dimer association into a hexamer and octamer occurred in parallel with a decrease of the affinity of essential SH-groups of cysteine for DTNB in 50% of the oligomeric molecule subunits. Thus, in the dimer two essential SH-groups were rapidly modified by DTNB at the same rate: k1 = k2 = (23.9 +/- 5.6).10(4) M-1 min-1. Within the hexamer, the rate of modification of 3 out of 6 SH-groups was practically unchanged: k1 = (10.6 +/- 2.3).10(4) M-1 min-1. Another 3 SH-groups in the remaining 50% of the subunits were partly masked, which manifested itself in a 10-fold decrease of their modification rate: k2 = (1.12 +/- 0.28).10(4) M-1 min-1. Within the octamer, the SH-groups rapidly interacted with DTNB only on 4 subunits: k1 = (20.7 +/- 2.2).10(4) M-1 min-1, whereas in the remaining 4 octamer subunits a practically complete masking of essential SH-groups was observed, as a result of which these groups became inaccessible to DTNB. This manifested itself in a 1000-fold decrease of the rate of SH-group modification by DTNB which reached that of non-essential SH-group modification. In has been found that a complete loss of the octamer activity is due to the modification of only 4 SH-groups which interact with DTNB at a high rate. A model for subunit association into a dimer, hexamer and octamer has been proposed. Presumably, 50% of the active centers in the mitochondrial creatine kinase octamer are not involved in the catalytic act.