Identification, total and differential counts of cell populations in guinea pig colostrum, milk and involution secretion were carried out. The cellular concentration in colostrum was high, but there was a dramatic decline in milk collected at the peak of lactation. Advancement of mammary gland involution was associated with a dramatic increase in cellular concentration. The neutrophil was the predominant cell type in colostrum, milk and early involution secretion. Macrophages were present at all stages of lactation and became the predominant cell as involution advanced. The proportion of lymphocytes was low and remained constant throughout the various stages of lactation. Small numbers of eosinophils and epithelial cells were present in most of the samples examined.