The incidences of Ig paraprotein (PP) and reticulum cell sarcomas (lymphomas) were studied in a group of 75 SJL mice, 9-11 months of age. PP and lymphoma were observed in 52% of the mice. PP alone was observed in an additional 27% and lymphoma alone in an additional 11% of mice. In attempts to establish a causal relationship between lymphomas and PP, three approaches were used: (a) Serum PP levels were followed during lymphoma growth in primary lymphoma bearing mice and found to decrease rather than to increase. (b) Recipients of primary transplants were examined for appearance of PP-related idiotypes (Id) in their sera and for lymphoma growth. The Id appeared early (Day 10-11) and then disappeared, while lymphoma growth usually was detectable by greater than or equal to 1 month. (c) One of the primary lymphomas was propagated as a tissue culture line and found not to produce any PP or intact Ig. It is concluded that the PP of SJL mice are not produced by their lymphomas. Other possible relationships are discussed, including the role of the PP as a symptom of a prelymphomatous stage that develops in a very high percentage of SJL mice.