A method of purifying K-252a characterized by treating microbial cells containing K-252a represented by formula (I) with an alkali solution to thereby convert the K-252a to K-252b represented by formula (II) or an alkali salt thereof to cause the same to be leached outside the microbial cells, methylating the K-252b or alkali salt thus leached to reconvert the same to K-252a, and isolating and recovering the K-252a.
