We attempted to evaluate the neonatal mouse model as an indicator of the virulence of staphylococcal strains freshly isolated from human patients in hospital. In preliminary studies with two previously characterised clinical isolates of Staphylococcus aureus and one of S. epidermidis, three indices of infection were studied. These were: mortality rate, multiplication of organisms in the skin, and the effect on weight gain. Of these inhibition of normal weight grain by mice given subcutaneous injections when 3 days old was the most convenient and easily applied test. At a challenge dose of 10(6) c.f.u., the multiplication of organisms in the skin was correlated with the inhibition of normal weight gain. Weight gain was used to compare the virulence of a small series of clinical isolates from different types of staphylococcal infection. Strains isolated from severe infections caused a greater inhibition of weight gain than did strains from milder infections or environmental sources.