The effects on the well-being of mice of feeding linoleic acid-enriched foodstuffs derived from ruminants receiving protected polyunsaturated oil were determined. Growth, reproductive productivity and longevity were compared in mice fed freeze-dried human diets containing either these products or the corresponding conventional ruminant-derived foods. A laboratory mouse pellet diet was used as a standard for the comparison. Growth rates and life spans were found to be similar in all three groups. Although the reproductive productivity of the mice on the polyunsaturated diet was never significantly different from that of mice on the conventional human diet, productivities of both groups on human diets were usually significantly below those of mice eating pellets. By the third generation of the second reproduction study, productivities of both these groups of mice had returned to the same level as those of mice eating pellets. Possible reasons for the depression in reproductive productivity in mice on the human diets are discussed. It was concluded that the polyunsaturated human diet neither impaired nor improved the lives of the animals eating the diet relative to the lives of animals eating the conventional human diet.