One hundred and twenty 4-month·old, single-combed, brown Hy-Line pullets were fed two isocaloric diets containing 16 or 13,5% protein with and without 5% clinoptilolite in four treatments with thirty hens per replicate. Sterile river sand was used as a filler in the other two diets not containing clinoptilolite so as to keep the diets isocaloric. The hens were housed in individual cages in a naturally ventilated laying house and fed the four different diets for twelve 28-day periods. No significant differences in live mass, egg mass, Haugh Units and shell thickness were observed between the four treatments. Significant treatment effects (P ≤0,05) were noticed in the days to first egg, number of eggs laid per hen, egg size percentages, feed consumed per hen and feed efficiency (kg feed/kg eggs). Highly significant differences (P ≤0,01) between treatments were found in faecal moisture percentage and bacterial colony counts in the proximal and distal ends of the intestine. Conflicting results between this experiment and others reported in the literature require further investigation.