The effect of synthetic β-carotene and of synthetic nucleotide bases was investigated in this experiment. Beginning one week prior to weaning, the diet fed to one experimental group of piglets was supplemented with 10% Rovimix β-carotene at 875 mg/kg diet (10 times the recommendation of the NRC, 1988). Synthetic uracil and adenine (98%, Sigma-Aldrich) were mixed into the diet for the other experimental group at quantities of 500 mg/kg diet of each substance. During the 3-week period of the experiment changes in the daily weight gain of the animals, their feed consumption and certain blood parameters were examined. Changes which could be observed with the progress of time in the haematological parameters (RBC, WBC, Hb, HTC, MCV, MCH, MCHC), in the ESR and in certain biochemical parameters (TP, ALB, CHOL, TRIG, UREA, ALT and AST) can be regarded as physiological. The LST values given for Phytohaemagglutinin and Concanavalin-A increased in the experimental groups by 50 and 130%, respectively (nucleotide), and by 60 and 30%, respectively, (carotene) respectively, while the blastic transformation of the lymphocytes did not vary in the control group. The supplements applied exerted no positive effect on in vivo cellular immune response. No significant degree of difference was observed between the plasma retinol concentrations determined at the end of the experimental period, (control: 376 ± 183, nucleotide: 412 ± 268 and carotene: 349 ± 251 µg/L) while there was a decrease in plasma E-vitamin concentration due to carotene supplementation (control: 5.58 ± 2.36, nucleotide: 5.83 ± 3.01 and carotene: 2.12 ± 1.60 mg/L).