A field experiment was conducted from 1991 to 1992 to examine induction and impact of hematopoietic neoplasia on the marine bivalve Mya arenaria in southeastern Massachusetts. Clams were collected from Little Buttermilk Bay and separated into three size classes (20-29, 30-39, and 40-49 mm shell length) in the laboratory. These sizes span the range of adults found in the population. A random subsample of these clams was taken to estimate disease prevalence at the start of the experiment, and this was found to be =10% in all size classes. Remaining clams were assigned randomly to two groups: Control and Treated. "Controls" were injected with filtered seawater, while "Treated" clams were injected with hemocytes extracted from diseased individuals. Injection of diseased hemocytes was performed to increase disease prevalence in the Treated group. Clams were returned to New Bedford Harbor, a more contaminated field location, where hematopoietic neoplasia is more prevalent, in January 1991, and characteristics of both groups were monitored for 555 days. Among Controls, probability of survival was size-dependent, with higher survival rates in larger clams. Treated clams had a lower probability of survival than Controls, and the magnitude of treatment effect increased with size class. The impact on survival was evident after 89 days, but it was first shown to be statistically significant after 189 days. Among Controls, probability of disease was strongly season-dependent, increasing in the large size class from 0.19 in spring to 0.50 in summer. During summer, Treated clams had a higher probability of being diseased than Controls. Among survivors, no significant sublethal effects due to treatment were detected in the field experiment. Experimental manipulation of disease prevalence may be a useful tool in future studies. In addition to results pertaining to disease, this study obtained long-term growth information, by size class, on somatic and reproductive tissue and shell size.