In comparison to 29 non bitten animals, severe tail biting was found to decrease the daily weight gain (DWG) by 25% in 8 fattening pigs during the period of biting. However, when comparing the weight gain of the lifetime between bitten and non bitten pigs, no influence of the tail biting was found. It is of interest that severely wounded pigs were parenterally treated with prokainpenicillin G for 3 consecutive days in connection with the tail biting, which could be suggested to promote the growth by reducing the influence of infections gained by the tail biting as well as of other infections present in herds rearing conventional pigs. Despite penicillin treatment, abscesses were more frequently recorded in tail bitten pigs than in non bitten animals. The tail biting was not equally distributed between the sexes, as barrows were more frequently bitten than gilts. Among the unbitten pigs, barrows were also found to grow faster than gilts. Indeed, when comparing tail bitten and non bitten barrows, a negative influence of tail biting on DWG was not only shown during the period of biting, but could also be monitored as a reduced DWG from that period until slaughter by 11% and during lifetime by 5% (the tail bitten gilts were too few to allow statistical calculations). These results clearly indicate that tail biting affects the growth rate of the lifetime despite penicillin treatment. However, it should be stressed that this decreased lifetime DWG may not be monitored when evaluating abattoir data because the sex distribution of the pigs may not be known in such materials.