Baardsen, A., Midtvedt, T. & Trippestad, A. Influence of methylprednisolone and azathioprine on polymorphonuclear neutrophils (PMN) and lymphocytes in germfree, monocontaminated and conventional rats. Acta path. microbiol. scand. Sect. C, 83: 210-214, 1975. In rats, protracted administration of methylprednisolone at high dose levels 1 ) increased the number of circulating PMN in germfree, monocontaminated (with Escherichia coli) and conventional rats. This drug-induced increase was more than doubled in the presence of microorganisms, as compared to germfree rats. 2) reduced the number of circulating lymphocytes more strongly in germfree rats than in monocontaminated and conventional rats. Protracted administration of azathioprine at high dose levels 1) reduced the number of circulating PMN in conventional rats, but not in germfree and monocontaminated rats. 2) did not affect the lymphocyte count at the dose level 40 mg per kg per day in any of the three groups of animals investigated.