X-irradiation of neonatal rats by low, fragmented and repeated doses reduces or eliminates several phosphoproteins (16, 18 and 19 Da) in an extract of the olfactory bulb. This coincides with a dramatic reduction of the number of GABAergic granule cells. The phosphorylation of these proteins was specifically stimulated in control animals by Ca(2+)/calmodulin, but not by cyclic adenosine triphosphate. The reduced inhibitory function, observed previously in other "granuloprived" bulbs, might be the consequence of loss of those cells expressing these low molecular weight phosphoproteins. To assess the cellular changes caused by X-irradiation, low molecular weight phosphoproteins can serve as specific markers in the olfactory bulb.