Myeloma cells secrete PTN into the supernatant during short-term culture. The myeloma cell lines MM-1S, U266, and the myeloma tumor LAGλ-1 were cultured for 48 hours and the culture supernatants analyzed for PTN protein concentration by ELISA. Concentration of PTN detected in the 3 myeloma cell lines was 0.5 ng/mL or less of PTN protein. Data are the average of experiments performed in triplicate.