Calcium was essential for the growth of A. vinelandii, A. chroococcum, A. beijerinckii and A. insigne in the presence and absence of combined nitrogen. A. agile and A. macrocytogenes were able to grow in the absence of calcium but growth was stimulated by the cation. Calcium could be replaced by strontium at roughly the same molar concentration. The authors present evidence suggesting that the calcium sparing effect of ammonium and sodium acetates reported in the literature may be dependent on the size of inoculum used.