The number of nodules produced per clover seedling inoculated with Rhizobium trifolii 0403 can be increased almost 2-fold by the addition of penicillin or mecillinam. Two-day-old dutch white clover seedlings grown in 250 milliliter boston round jars containing agar-solidified plant growth medium were inoculated with exponentially growing Rhizobium trifolii 0403 cells. Penicillin or mecillinam (100 micrograms per milliliter) were added immediately or after 24 hours. Following 42 days growth, 10 replicate sets of 5 plants for each treatment were assayed for nodule number, plant dry weight, and Kjeldahl nitrogen. Both antibiotics increased nodule number, plant dry weight, and Kjeldahl nitrogen. Increases in nodule number and dry weight were statistically significant. The range of values in Kjeldahl nitrogen was so extensive as to make the data insignificant at the P < 0.05 level, however nodule number, plant dry weight, and Kjeldahl nitrogen displayed a significant correlation with each other. There were no significant differences in treatment with either antibiotic or with time of treatment. Nodule number increased by about 85%, and plant dry weight and nitrogen increased by about 30%.