Immune responsiveness, the ability of an organism to effectively respond immunologically following antigenic exposure, is an essential component of life history, as organisms require effective immune functionality in order to grow, survive and reproduce. However, immune status is also associated with concomitant trade-offs in these physiological functions. Herein we demonstrate the validation of phytohaemagglutinin (PHA) injection in saltwater crocodiles, Crocodylus porosus, to assess cellular immune responsiveness. Following injection of 2 mg mL^–1 PHA into the hind toe webbing, we observed a peak swelling response 12 h after injection, with PHA inducing increased thickness compared with webs injected with phosphate-buffered saline (PBS) (F_5,518 = 145.13, P < 0.001). Subsequent injections increased responsiveness relative to the primary injection response (F_5,290 = 2.92, P = 0.029), suggesting that PHA exposure induced immunological memory, a tenet of acquired immunity. Histological examination revealed that PHA-injected toe webs displayed increased numbers of leukocytes (granulocytes, macrophages, and lymphocytes) relative to PBS-injected webs, with peak leukocytic infiltrate observed 12 h after injection. We suggest the use of PHA injection in crocodilians as a measure of cellular immune responsiveness in agricultural (production and animal welfare), ecological, and toxicological contexts.