This paper demonstrates that vitamin A deficient (A-) cotton rats are more susceptible to infection with Litomosoides carinii than normal (A+) cotton rats. However, embryogenesis is frequently retarded in female worms from A- hosts. Groups of young cotton rats were depleted of their liver stores of vitamin A by feeding them a vitamin A free diet from weaning. From 6 weeks of age this diet was supplemented with either 2, 5, 10, 25 or 100 IU vitamin A per 100 g body weight per day and half the animals in each group were infected with L. carinii. At autopsy, 51 days post-infection, serum vitamin A levels were proportional to vitamin A intake but were unaffected by infection (P less than 0.05). A+ cotton rats grew more rapidly and gained more weight than A- ones; infected animals gained less weight than their uninfected controls. More parasites developed in rats fed either 2 or 5 IU vitamin A than those fed either 10, 25 or 100 IU (P less than 0.02) and parasites from the 5 IU group were significantly longer than those from all other groups (P less than 0.02). However, microfilarial development was retarded in female worms from A- animals as compared with A+ ones.