On incubation with silicate and a suitable substrate, cellfree extracts of Proteus mirabilis incorporate the anion Si, which is successively destributed and linked to various compounds. The distribution pattern changes with increasing incubation time: On fractionation with alcohol-ether (AES), most of the Si appears first in the alcohol-ether-insoluble (AEI) fraction reaching a maximum after 2 hours, whereafter a steady decline of the Si-concentration is observed. This fraction was found to contain water-extractable Si-compounds. The amount of Si that can be extracted from the precipitated material increases with time and temperature; maximum yield is obtained extracting the residue at 50 °C for 3 hours. The amount of waterextractable Si-compounds present in the AEI-fraction also varies with increasing incubation time: The high concentration found after 30 min rapidly declines to a minimum after 4 hours, followed by a slow increase during further incubation. Both the AEI- and AES-fraction contain acid-labile Si-compounds, liberating free silicic acid on acid hydrolysis. The percentage of these acid-labile compounds in both fractions was also found to vary with the time of incubation.