Although many antimicrobial agents have been reported to cause immunosuppression in animals, macrolide antibiotics enhance immune function. Tylosin is a macrolide antibiotic approved for the control of mycoplasmosis in poultry. The purpose of this investigation was to determine the effect of tylosin on cellular immune functions in chickens. There was no significant difference in adherent splenocyte chemotaxis between tylosin-treated and untreated (control) chickens. Tylosin increased splenocyte proliferation and splenocyte conditioned medium (CM) proliferative activity above control levels. Removal of adherent splenocytes before preparation of CM caused a reduction in CM proliferative activity. Tylosin also increased antitumor activity of splenocytes. These data are the first to suggest that the macrolide antibiotic, tylosin tartrate, has a modulatory effect in chickens on the immune parameters studied.