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SUMMARY: Myxamoebae of Dictyostelium discoideum strain Ax-2 were adapted to growth in a medium containing B vitamins, a vitamin-free tryptic digest of casein, soluble starch and inorganic salts. Omission of vitamins from this medium confirmed that d-biotin, thiamin, folate and a-lipoate are essential for myxamoebal growth and showed that riboflavin or flavin mononucleotide is also required. No requirement for other vitamins or growth factors could be detected. A defined medium was developed by replacement of the tryptic digest of casein with a mixture of 20 l-amino acids and addition of Tween 80. Tween 80 caused starch precipitation in the medium and this seemed essential for stimulating growth of the myxamoebae.