Fifteen athymic rat strains from 11 breeding colonies were housed within an experimental facility for an immunological study. Health status records supplied with 14 of the strains listed infections by Kilham's rat virus (KRV), Clostridium piliforme (Bacillus piliformis) and Pasteurella pneumotropica for 2, 2 and 1 colonies respectively. In sera taken previous to the study from euthymic rats of 10 strains, antibodies to KRV were detected in 3 strains, to Pneumonia virus of mice (PVM), Rat corona virus (RCV) and Sendai virus in one strain each and to P. pneumotropica in 2 strains. Only 2 of the KRV infections had been reported by the supplier. During the study rats of all 10 strains developed antibodies to 2-4 of viral antigens. Eight out of 10 rat strains seroconverted to 1-5 of the antigens C. piliforme (B. piliformis), Bordetella bronchiseptica, Haemophilus spp., P. pneumotropica and Streptobacillus moniliformis. Two rat strains housed in filtertop cages did not develop antibodies to bacterial antigens. The potential detrimental effects of intercurrent infections on the outcome of the comparative immunological study are discussed.