The lymphocyte stimulation test (LST) is an in vitro corollary of in vivo cell‐mediated immunity. We have analyzed LST results from 32 dogs covering a 16‐month time span. Two mitogens (plant lectins) were used in the LST, phytohemagglutinin (PHA) and concanavalin A (Con A) with comparable results. All dogs exhibited a seasonal variation in LST results using both mitogens. The dogs were either controls or were in one of five groups that received whole‐body gamma ray irradiation from a 60CO source. There was little effect of irradiation except on the mean levels of oscillation. Few differences were noted among groups. The estimated mean times of peak activity in the LST ranged from July 19‐August 2 for PHA and from July 18‐August 15 for Con A. The times of peak of individual dogs for both mitogens were predominantly in the calendar quarter June 21 ‐September 20, and were, therefore, not randomly distributed throughout the year. The implications of these findings are discussed.