The relationship between serum protein, immunoglobulin concentrations and protein molecular weight profile (PMWP) alterations of sea bass (131.3 ± 4.3 g) reared in sea water with sublethal concentrations of ammonia was studied over two periods totalling 116 days. During the exposure period (62 days) the first group (group 1) lived in sea water with 0.204 mg·L−1 unionized ammonia nitrogen equivalent to 12 % of the lethal concentration for 50 % of a population exposed for 96 h to ammonia (96-h LC50), whilst the second group (group 2) lived in sea water with 0.340 mg·L−1 UIA-N equivalent to 20 % of the 96-h LC50 of ammonia. Then, the two groups were left for a recovery period (54 days) in the same water as the control group (group 0). The determination of the total immunoglobulin (Ig) concentration was carried out by enzyme-linked immunosorbant assay (ELISA). Gel filtration columns were used for the serum PMWP. Serum Ig concentration of the exposed fish (group 2: 1.76 ± 0.43 mg·mL−1; group 1: 1.19 ± 0.33 mg·mL−1) was lower than the control group 13.39 ± 1.01 mg·mL−1) after 21 days of exposure period but this difference was reduced at the end of this period (day 62) and treated group Ig concentrations switched to higher than the control group after the recovery period (day 116, group 2: 9.75 ± 1.84 mg·mL−1; group 1: 7.50 ± 1.22 mg·mL−1 group 0: 6.38 ± 1.13 mg·mL−1). In fact, at the end of the experiment, the cumulative Ig production difference between fish exposed to ammonia and the control was less than 10%. Although a similar evolution of the Ig serum occurred with protein concentration, the serum protein concentration deficit of group 1 was restored at the end of the exposure period (group 2: 46.49 ± 2.34 mg·mL-1; group 0: 46.74 ± 1.97 mg·mL−1) and the cumulative production during the experiment was not significantly different between group 1 and group 0. However, this remained lower for group 2. During the exposure period, the PMWP of treated fish moved towards smaller molecular weight proteins. This alteration of the PMWP showed that the 0.2-kDa fraction increased, and another fraction appeared at the end of the exposure period. However, at the end of the recovery period, no difference was found between the PMWPs of control and treated fish. The alterations of the serum protein and Ig of fish reared, for a limited period (2 months), in water with sublethal concentrations of ammonia (20 % 96-h LC50 or less) should disappear completely after a few months in normal rearing conditions.