Victorian trainer Archie Alexander is under investigation after one of his stable's horses returned an illegal cobalt level after a win at Kilmore.

Racing Analytical Services Limited reported a test from Tom's Knight after his win in a maiden on November 2 detected cobalt at a mass concentration of 141 micrograms per litre in urine with the reserve sample showing 137 mcg/L.

The threshold is 100 mcg/L.

Stewards initially told Alexander of the irregularity on November 28 and RV said on Monday the investigation was continuing,

Cobalt is an essential mineral nutrient which affects multiple body systems and which can be toxic at high dosages.