A sensitive spectrophotometric method for the determination of cobalt(II) using a newly synthesized chromogenic reagent N-(2-pyrrolylmethylene}-2,4,6-triamino-l,3,S-triazine (PMTT) has been developed. The optimum conditions for the reaction of PMTT with cobalt(II) were studied. The calibration graphs were plotted. The molar absorptivity was found to be 1.83xI05 dm3 mol-1 cm-l. To check the validity and sensitivity of PMTT, the method was applied for the determination of cobalt (II) in different samples [tablet, capsule and injection formulations] of vitamin B12. The quantitative results of these studies show the presence of Co(II) as 0.84, 0.82 and 0.81 ppm, respectively, in tablet, capsule and injection formulations in the tested vitamin. B12 samples.