Leukocytes from 1 ml of blood from cattle seropositive to the bovine leukemia virus were cultured for 3 days and then tested by radioimmunoassay for antigen production. Infectivity of blood from each animal was also tested by calf inoculation and subsequent serologic detection of bovine leukemia virus transmission by agar-gel immunodiffusion. In a preliminary experiment, blood from each of 3 antigen-positive cattle was inoculated intracutaneously into 2 calves in volumes of 20 or 100 microliter. Blood from each of 4 antigen-negative cattle was similarly inoculated into 3 calves in volumes of 20, 100, or 500 microliter. At the termination of the experiment (8 weeks after inoculation), all 6 calves given blood from antigen-positive cattle had seroconverted, and 11 of 12 calves given blood from antigen-negative cattle had seroconverted. In a 2nd experiment, blood from each of 2 antigen-positive and 2 antigen-negative cattle was inoculated into pairs of calves in volumes of 1, 10, or 20 microliter. At the end of the experiment (12 weeks after inoculation), all calves inoculated with blood from antigen-positive cattle had seroconverted, but only 6 of 12 calves that had been given blood from antigen-negative cattle had seroconverted. The relative infectivity of blood was best illustrated by comparing results from the 1-microliter inoculations. At that volume, the 4 calves given blood from antigen-positive cattle were infected, whereas none of 4 calves given blood from antigen-negative cattle was infected.