Optically pure D-lactic acid was produced by fermentation of lactose with Lactobacillus bulgaricus Lb-12, and purified by crystallisation as magnesium D-lactate followed by extraction with butanol. The yield of D-lactate and contaminations with nitrogen and phosphorus were mapped during the purification procedure. The overall yield of D-lactic acid was 72% and the purity was more than 99%. Contaminations in the final D-lactic acid with nitrogen, phosphorus and L-lactic acid were only 0.032% w/w, 0.018% w/w and 0.04% w/w respectively.